Medicinal values of garlic: A review

Abstract

Garlic products are used as sources of medicine in many ways in human beings in their day today life. As a result, researchers from various disciplines are now directing their efforts towards discovering the medicinal values of garlic on human health. The main interest of researchers in the medicinal values of garlic is its broad-spectrum therapeutic effect with minimal toxicity. Garlic extract has antimicrobial activity against many genera of bacteria, fungi and viruses. Garlic contains a higher concentration of sulfur compounds which are responsible for its medicinal effects. The chemical constituents of garlic have also been investigated for treatment of cardiovascular disease, cancer, diabetes, blood pressure, atherosclerosis and hyperlipidaemia and highly praised by several authors. Therefore, this paper is reviewed to inspire and impress the young researchers about the medicinal values of garlic.

Full text

Vol. 5(9), pp. 401-408, September, 2013
DOI: 10.5897/IJMMS2013.0960
ISSN 2006-9723 ©2013 Academic Journals
http://www.academicjournals.org/IJMMS
International Journal of Medicine
and Medical Sciences
Review
Medicinal values of garlic: A review
Gebreselema Gebreyohannes1* and Mebrahtu Gebreyohannes2
1Department of Biology, Faculty of Natural and Computational Sciences, University of Gondar, Ethiopia.
2Faculty of Veterinary Medicine, University of Gondar, Ethiopia.
Accepted 12 August, 2013
Garlic products are used as sources of medicine in many ways in human beings in their day today life.
As a result, researchers from various disciplines are now directing their efforts towards discovering the
medicinal values of garlic on human health. The main interest of researchers in the medicinal values of
garlic is its broad-spectrum therapeutic effect with minimal toxicity. Garlic extract has antimicrobial
activity against many genera of bacteria, fungi and viruses. Garlic contains a higher concentration of
sulfur compounds which are responsible for its medicinal effects. The chemical constituents of garlic
have also been investigated for treatment of cardiovascular disease, cancer, diabetes, blood pressure,
atherosclerosis and hyperlipidaemia and highly praised by several authors. Therefore, this paper is
reviewed to inspire and impress the young researchers about the medicinal values of garlic.
Key words: Allium sativum, immunity booster, antibacterial, antifungal, antiviral, anticancer
INTRODUCTION
Natural products of animals, plants and microbial sources
have been used by man for thousands of years either in
the pure forms or crude extracts to treat many diseases
(Parekh and Chanda, 2007). Garlic (Allium sativum L.) is
one of those plants that were seriously investigated over
several years and used for centuries to fight infectious
diseases (Onyeagba et al., 2004). The taxonomic posi-
tion of garlic and related genera had been a matter of
controversy for long period of time. The most recent
classification scheme of garlic was class Liliopsida,
subclass Liliidae, superorder Liliianae, order Amary-
llidales, family Alliaceae, subfamily Allioideae, tribe
Allieae and genus Allium which is mainly based on the
sequences of nuclear ribosomal DNA (Friesen et al.,
2006).
The early Egyptians used garlic to treat diarrhea and its
medical power was described on the walls of ancient
temples and on papyrus dating to 1500 BC (Bradley,
1992). It was used by Greek physicians Hippocrates and
Galen to treat intestinal and extra-intestinal diseases;
ancient Japanese and Chinese used it to treat headache,
flu, sore throat and fever. In Africa, particularly in Nigeria,
it is used to treat abdominal discomfort, diarrhea, otitis
media and respiratory tract infections (Jaber and Al-
Mossawi, 2007). In Europe and India, it was used to treat
common colds, hay fever and asthma. Garlic is
nicknamed as Russian penicillin for its widespread use as
a topical and systemic antimicrobial agent; it is commonly
used in many cultures as an excitement and reputation of
healing power (Timbo et al., 2006).
POTENTIALLY ACTIVE CHEMICAL CONSTITUENTS
OF GARLIC
Garlic contains at least 33 sulfur compounds, several
enzymes and the minerals germanium, calcium, copper,
iron, potassium, magnesium, selenium and zinc; vitamins
A, B1 and C, fiber and water. It also contains 17 amino
acids to be found in garlic: lysine, histidine, arginine,
aspartic acid threonine, swine, glutamine, proline,
glycine, alanine, cysteine, valine, methionine, isoleucine,
leucine, tryptophan and phenylalanine (Josling, 2005). It
has a higher concentration of sulfur compounds than any
other Allium species which are responsible both for
garlic’s pungent odor and many of its medicinal
*Corresponding author. E-mail: ggebreselema@yahoo.com.

402 Int. J. Med. Med. Sci.
effects. One of the most biologically active compounds in
garlic is allicin (diallyl thiosulfinate or diallyldisulfide). The
most abundant sulfur compound in garlic is alliin (S-
allylcysteine sulfoxide), which is present at 10 and 30
mg/g in fresh and dry garlic, respectively (Lawson, 1998).
Typical garlic food preparation such as chopping, mincing
and crushing disturbs S-allyl cysteine sulfoxide and
exposed it to the allinase enzymes, then quickly
converted it to diallyl thiosulfinate, which give off garlic’s
characteristic aroma. The allinase enzyme responsible
for diallyl thiosulfanate conversion becomes inactivated
below a pH of 3.5 or with heating (Pedrazza-Chaverri et
al., 2006). Although allicin is considered the major
antioxidant and scavenging compound, recent studies
showing that other compounds may play stronger roles;
such as polar compounds of phenolic and steroidal origin,
which offer various pharmacological properties without
odor and are also heat stable (Lanzotti, 2006).
ROLE OF GARLIC IN HEALTH
Garlic can rightfully be called one of nature’s wonderful
plants with healing power. It can inhibit and kill bacteria,
fungi, lower (blood pressure, blood cholesterol and blood
sugar), prevent blood clotting, and contains anti-tumor
properties. It can also boost the immune system to fight
off potential disease and maintain health (Abdullah et al.,
1988). It has the ability to stimulate the lymphatic system
which expedites the removal of waste products from the
body. It is also considered an effective antioxidant to
protect cells against free radical damage. It can help to
prevent some forms of cancer, heart disease, strokes and
viral infections. Garlic alone can provide us with over two
hundred unusual chemicals that have the capability of
protecting the human body from a wide variety of
diseases. The sulfur containing compounds found in
garlic afford the human body with protection by
stimulating the production of certain beneficial enzymes
(Mansell and Reckless, 1991).
Treat cardiovascular disease
Disorders of the heart and the circulatory system claim
more lives than any other diseases. It is the obstruction
or clogging of the coronary arteries which causes more
deaths than any other factors. The arteries, which supply
the heart with blood and oxygen, become increasingly
narrower as plaque builds up over time. When blood
supply becomes restricted, a certain portion of the heart
is deprived of oxygen and leads to heart attack. The two
greatest means of heart disease are high blood pressure
and high blood serum cholesterol levels; which are
directly impacted by the therapeutic action of garlic. The
relevant role of garlic in coronary heart disease was done
on rabbits and found that even pre-existing athero-
sclerotic deposits and lesions could actually be reversed
if garlic was consistently consumed (Bordia, 1981).
From a study conducted in India, 432 coronary artery
patients were randomly grouped into two groups and half
of them were supplied with garlic juice in milk, whereas
the other group patients were not supplied with garlic
juice. The result showed that within the three years of the
study time, nearly twice as many patients had died in the
group not supplied with garlic juice (Yeh et al., 2006). It is
well reported to scavenge oxidants, increase superoxide
dismutase, catalase, glutathione peroxidase, glutathione
levels, inhibit lipid peroxidation as well as it reduces
cholesterol synthesis by inhibiting 3-hydroxy-3-
methylglutaryl-CoA. It has been shown to reduce platelet
aggregation, arterial plaque formation, decrease
homocysteine, lower blood pressure, and increase
microcirculation. It may also help prevent cognitive
decline by protecting neurons from neurotoxicity and
apoptosis, thereby preventing ischaemia or reperfusion-
related neuronal death and by improving learning and
memory retention (Borek, 2006).
Reduces high blood pressure/hypertension
Garlic has probably been most popularized as a com-
plementary therapy for blood pressure control (Capraz et
al., 2006). A recent in vitro study has confirmed that, the
vasoactive ability of garlic sulfur compounds whereby red
blood cells convert garlic organic polysulfides into
hydrogen sulfide, a known endogenous cardio-protective
vascular cell signaling molecule (Benavides et al., 2007).
Using 2400 mg garlic tablet containing 31.2 mg allicin has
high dose reduced diastolic pressure by 16 mmHg after 5
h of administration (McMahon and Vargas, 1993). A
meta-analysis made on pooled data from 415 patients
showed also reduction of 7.7 mmHg diastolic pressure
(Silagy and Neil, 1994).
As natural blood thinner
Platelets and fibrin play great role in blood clotting and
higher amount of fibrin in blood can cause heart attack.
Garlic constituents can reduce fibrin formation and also
help reduce the fibrin existing in the blood even better
than aspirin (Fukao et al., 2007). Ajoene, a sulfur
compound found in garlic seems to be responsible for its
anti-clotting effect; but ajoene is only viable at room
temperature or above, it is not present in raw or freeze-
dried garlic. It is believed that the addition of garlic to a
diet can help to increase the breakdown of fibrin from 24
to 30% in people (Ernst, 1994).
As natural immunity booster
With the arrival of frightening viral diseases like
HIV/AIDS, boosting immunity system is receiving a new
attention. Because these types of diseases have no

effective cures or treatments, strengthening the body’s
ability to fight off infection has become even more
important. Garlic has abundant sulfur containing amino
acids and other compounds that seem to initiate
increased activity in the immune system (Lau et al.,
1991). It is one of the impressive conductors of the
body’s immune system; which stimulates immune
function by making macrophages or killer cells more
active. We are constantly beaten by inadequate nutrition,
cigarette smoke, physical injury, mental tension and
chemical pollution. In light of the enormous pressures,
which our immune systems sustain, supplemental
nutrients like garlic are clearly needed (Salman et al.,
1999). Its remarkable content of germanium alone offers
excellent immune stimulation. In addition to germanium,
garlic contains thiamine, sulfur, niacin, phosphorous, and
selenium (Morioka et al., 1993).
Preliminary studies in humans, using an alliin standar-
dized garlic powder preparation, have demonstrated
positive effects on immunoreactions and phagocytosis. In
aged subjects, the administration of 600 mg garlic
powder per day for 3 months induced significant (p<0.01)
increases in the percentage of phagocytosing peripheral
granulocytes and monocytes when tested ex vivo for their
ability to engulf Escherichia coli bacteria. Another human
study was conducted with an unrefined garlic extract (5 to
10 g/day) which was given to HIV/AIDS patients. For the
seven patients who completed the 12 weeks study, there
was a major increase in the natural killer cells activity
from a seriously low mean value (Abdullah et al., 1988).
In USA, trials in HIV/AIDS patients have demonstrated
the enhancement of natural killer cells activity using garlic
extracts; and Chinese studies with viral infections in bone
marrow transplant patients have demonstrated a “potent
antiviral activity”. A double blind placebo controlled
survey using a 100% allicin yielding supplement has
reported that allicin can reduce the occurrence of the
common cold and recovered from symptoms (Josling,
2001).
Atherosclerosis and hyperlipidaemia
Health claims advertizing garlic’s universal ability to lower
cholesterol level and decrease lipid peroxidation in order
to inhibit plaque formation. In vitro studies clearly have
shown that, it has an ability to suppress low density
lipoprotein (LDL) and an increased resistance of LDL to
oxidation (Lau, 2006). Results from controlled human
studies are mixed with studies performed in the early
1990’s and was showing effective results. As more
researches were conducted newer processes to extract
garlic, recent study of 15 hypercholesterolemia patients
evaluated a material produced from garlic fermented with
the mold Monascus pilosus. This preparation significantly
reduced serum total cholesterol and low density
lipoprotein cholesterol levels when checked at 2 and 4
Gebreyohannes and Gebreyohannes 403
weeks after treatment beginning. The level of
triglycerides had a tendency towards reduction in hyper-
triglycerdemic patients as well, whereas high density
lipoprotein cholesterol was unchanged (Sumioka et al.,
2006). After 60 days of supplementation, low-density
lipoprotein, serum triglyceride and very low density
lipoprotein, were reduced by 21, 37, and 36.7%,
respectively (Jeyaraj et al., 2006).
Prevents diabetes
A number of animal studies support the effectiveness of
garlic in reducing blood glucose in streptozotocin-induced
as well as alloxan-induced diabetes mellitus in mice.
Most of the studies showed that garlic can reduce blood
glucose level in diabetic mice and rabbits (Ohaeri, 2001).
A study was conducted to evaluate oral administration of
garlic extract for 14 days on the level of serum glucose,
total cholesterol, triglycerides, urea and uric acid, in
normal and streptozotocin-induced diabetic mice. The
result of the study showed significant decrease (p<0.05)
in serum glucose, total cholesterol, triglycerides, urea,
uric acid, aspartate amino transferase and alanine amino
transferase levels, while increased serum insulin in
diabetic mice, but not in normal mice. From a comparison
study made between the action of garlic extract and
glibenclamide, it was shown that the antidiabetic effect of
the garlic was more effective than the glibenclamide (Eidi
et al., 2006).
Anticancer
Of the many favorable actions of garlic, inhibition of the
growth of cancer is perhaps the most prominent. It has
several synergistic effects that either prevent or possibly
may fight cancer. The action of garlic has been attributed
to stimulate immune effector cells including T-cell and
natural killer cells. Numerous epidemiological, clinical and
laboratory studies have demonstrated that, garlic has a
great role in cancer prevention especially in relation to
digestive tract cancers. Human population studies have
shown that, regular intake of garlic reduces the risk of
esophageal, stomach and colon cancer. This was thought
to be due to the antioxidant effect of allicin in reducing the
formation of carcinogenic compounds in the gastro-
intestinal tract (Galeone et al., 2006).
Dutch research in the Netherlands cohort study found a
significant decrease in the development of stomach
cancer in those consuming garlic close relatives of onions
(Dorant et al., 1996). Garlic reduces the risk of patients
with prostate cancer, especially those with localized
disease. Men in the higher of two intake categories of
total Allium vegetables (>10.0 g/day) had a statistically
significant lower risk of prostate cancer than those in the
lowest category (<2.2 g/day). Similar comparisons

404 Int. J. Med. Med. Sci.
between categories showed reductions in risk for men in
the highest intake categories for garlic specifically. The
reduced risk of prostate cancer was independent of body
size, intake of other foods and total calorie intake and
was more pronounced for men with localized prostate
cancer than with advanced prostate cancer (Hsing et al.,
2002). Prostate specific antigen serum markers had
significant decreases during short term ingestion, but
returned to baseline after 4 weeks (Mehraban et al.,
2006).
A very important epidemiological study for Americans
has been published in which the intake of 127 foods
(including 44 vegetables and fruits) was determined in
41,387 women (ages 55 to 69) followed by a five year
monitoring of colon cancer incidence. The most striking
result of this “Iowa Women’s Health Study” was the
finding that garlic was the only food which showed a
statistically significant association with decreased colon
cancer risk. For cancers anywhere in the colon, the
modest consumption of one or more servings of garlic
(fresh or powdered) per week resulted in a 35% lower
risk, while a 50% lower risk was found for cancer of the
distal colon (Steinmetz et al., 1994).
Dermatologic applications
A study examined 43 persons for their topical use of two
different garlic extracts for wart and corn treatment. Of
these persons, 15 volunteers utilized a water extract of
garlic, while 23 volunteers applied lipid extract to
appropriate areas twice a day. Five controls applied only
a neutral solvent. All lipid extract volunteers experienced
complete resolution of wart and 80% of corn within one to
two weeks. The water extract seemed to be less potent,
with complete dissolution of smaller warts and corns, and
only partial dissolution of larger ones. Controls showed
no improvement from baseline. The lipid extract did
cause some burning, redness, blistering and skin darken-
ing, which was resolved after conclusion of use
(Dehghani et al., 2005).
Antimicrobial
The antimicrobial properties of garlic were first described
by Pasteur (1958), and since then, many researches had
demonstrated its effectiveness and broad spectrum
antimicrobial activity against many species of bacteria,
viruses, parasites, protozoan and fungi (Jaber and Al-
Mossawi, 2007). Garlic is more effective with least side
effects as compared to commercial antibiotics; as a
result, they are used as an alternative remedy for
treatment of various infections (Tepe et al., 2004). Out of
the many medicinal plants, garlic has an antimicrobial
property which protects the host from other pathogens
highlighting the importance of search for natural
antimicrobial drugs (Bajpai et al., 2005; Wojdylo et al.,
2007). Previously conducted researches confirmed that
garlic is not only effective against Gram positive and
Gram negative bacteria but also possess antiviral and
antifungal activities (Tsao and Yin, 2001).
Antiviral
Garlic and its sulfur constituents verified antiviral activity
against coxsackievirus species, herpes simplex virus
types 1 and 2, influenza B, para-influenza virus type 3,
vaccinia virus, vesicular stomatitis virus, human
immunodeficiency virus type 1 and human rhinovirus type
2. The order of compounds found in garlic for virucidal
activity was, ajoene > allicin > allyl methyl thiosulfanate >
methyl allyl thiosulfanate; no activity was found for the
polar fractions, alliin, deoxyalliin, diallyl disulfide, or diallyl
trisulfide. Several laboratory tests have shown that garlic
is an effectual treatment for both the influenza B virus
and herpes simplex virus. Two independent researchers
in Japan and Romania have found that garlic is able to
protect living organisms from the influenza virus (Tsai et
al., 1985). Most recently, a double blind placebo con-
trolled study has shown significant protection from the
common cold virus. As conducted by The Garlic Centre,
published in Advances in Therapy, this is the first serious
work to show prevention, treatment and reduction of re-
infection benefits from taking Allimax Powder capsules
once daily (Josling, 2001).
Antibacterial
Garlic extract inhibits the growth of Gram positive and
Gram negative bacteria, such as Staphylococcus,
Streptococcus, Micrococcus, Enterobacter, Escherichia,
Klebsiella, Lactobacillus, Pseudomonas, Shigella,
Salmonella, Proteus, and Helicobacter pylori (Tsao and
Yin, 2001). Its antibacterial activity is mainly due to the
presence of allicin produced by the enzymatic activity of
allinase on alliin. Allicin is considered to be the most
potent antibacterial agent in crushed garlic extracts, but it
can be unstable, breaking down within 16 h at 23°C
(Hahn, 1996). However, the use of a water-based extract
of allicin stabilizes the allicin molecule due to the
hydrogen bonding of water to the reactive oxygen atom in
allicin or there may be water soluble components in
crushed garlic that destabilize the molecule (Lawson,
1996). The disadvantage of this approach is that allicin
can react with water to form diallyl disulphide, which does
not exhibit the same level of antibacterial activity of allicin
(Lawson and Wang, 1996).
Antifungal
Ajoene is an active compound found in garlic which plays
a great role as topical antifungal agent (Ledezma and

Apitz-Castro, 2006). Garlic has been shown to inhibit
growth of fungal diseases as equally as the drug
ketoconazole, when tested on the fungi Malassezia furfur,
Candida albicans, Aspergillus, Cryptococcus and other
Candida species (Shams-Ghahfarokhi et al., 2006). A
report from a Chinese medical journal delineates the use
of intravenous garlic to treat a potentially fatal and rare
fungal infection of the brain called Cryptococcus
meningitis. In the report, the Chinese compared the effec-
tiveness of the garlic with standard medical treatment
which involved a very toxic antibiotic called Amphotericin-
B. The study revealed that, intravenous garlic was more
effective than the drug and was not toxic regardless of its
dosage (Lemar et al., 2007).
A study found that Candida colonies were substantially
reduced in mice that had been treated using liquid garlic
extract. The study also revealed that garlic stimulated
phagocytic activity. This implies that infections such as
Candida may be controlled because garlic stimulates the
body’s own defenses. Garlic oil can be used to treat ring-
worm, skin parasites and warts if it is applied externally.
Lesions that were caused by skin fungi in rabbits and
guinea pigs were treated with external applications of
garlic extract and began to heal after seven days
(Sabitha et al., 2005).
Antiparasitic
Many herbalists worldwide recommend garlic as a treat-
ment for intestinal parasites. In some cultures, children
infested with helminthes are treated with enemas
containing crushed garlic. One of the traditional Chinese
medical treatments for intestinal diseases is an alcoholic
extract of crushed garlic cloves. Allicin exhibits anti-
parasitic activity against major human intestinal parasites
such as Entamoeba histolytica, Ascaris lumbricoides and
Giardia lamblia (Kalyesa et al., 1975). Entamoeba
histolytica, the human intestinal protozoan parasite, is
very sensitive to allicin, as only 30 μg/ml of allicin totally
inhibits the growth of amoeba cultures (Mirelman et al.,
1987). Moreover, researchers have found that at lower
concentrations (5 μg/ml), allicin inhibited 90% the
virulence of trophozoites of E. histolytica as determined
by their inability to destroy mono-layers of tissue-cultured
mammalian cells in vitro (Ankri et al., 1997).
Role of garlic against multi-drug resistant bacteria
Garlic is active against microorganisms that are resistant
to antibiotics and the combination of garlic extracts with
antibiotics leads to partial and total synergism (Didry et
al., 1992). The emergence of multi-drug resistant strains
of Gram negative (Pseudomonas, Klebsiella, Entero-
bacter, Acinetobacter, Salmonella species, etc) and
Gram positive (Staphylococcus, Enterococcus, Strepto-
coccus species, etc) bacteria is troubling for human and
Gebreyohannes and Gebreyohannes 405
animals. The emergence of epidemic methicillin resistant
Staphylococcus aureus (MRSA) resistant to mupirocin
has led many authors to suggest that the use of
mupirocin should be controlled more strictly, especially as
there is a lack of alternative agents. Consequently, garlic
is an alternative agent for the treatment of MRSA and in a
great demand (Sharma et al., 2005).
Role of garlic against multi-drug resistant
tuberculosis (MDR-TB)
Scientific evidence from randomized clinical trials
supports the use of garlic and enhances access for MDR-
TB infected people, through the public health system. Its
use can allow an effective MDR-TB management, due to
its affordability and the absence of toxic effects (Catia et
al., 2011). In view of the increased incidence of MDR-TB,
the research of new anti-tubercular drugs based on
affordable and more effective treatments has already
begun. Studies on innovative alternative plant extracts of
medicinal values need to be emphasized, as plants are
an important source of new antimicrobial agents, with
little toxicity, able to replace drugs to which Myco-
bacterium resistance has occurred (Amin et al., 2009).
As garlic is concerned, the in vitro tests undertaken
about the inhibitory effect on MDR-TB are at an
advanced stage whereas few researches in vivo have
been conducted. The concentration of garlic extract re-
quired was in the range of 1.34 to 3.35 mg/ml suggesting
that there is only a slight variation in the susceptibility of
the strains to allicin (Delaha and Garagusi, 1985). The
anti-tuberculosis activity in vivo of garlic oil preparation
was demonstrated in a study of guinea pigs which were
given an intra-peritoneal dose of 0.5 mg/kg. However,
when garlic oil was used, a reduced causative process
was noted in the organs involved, indicating that garlic oil
administration causes less marked lesions in the viscera
of the animals inoculated with tubercle bacilli (Jain,
1998). The high potential of garlic extract was revealed to
inhibit the growth of Mycobacterium tuberculosis H37Rv
and M. tuberculosis TRC-C1193, susceptible and resis-
tant to isoniazid (first-line anti-tuberculosis medication),
respectively. The minimum inhibitory concentration (MIC)
of garlic was between 80 and 160 μg/ml for the
susceptible strain and 100 and 200 μg/ml for the resistant
strain. In addition, water extract of garlic was proven to
inhibit the incorporation of 14C glycine into the whole
cells, indicating that the primary mechanism of action is
by inhibition of protein synthesis (Ratnakar and Murthy,
1996).
An interesting in vitro test about the anti-tubercular
activity of garlic was performed in Nigeria using disc
diffusion method and compared with standard antibiotics.
The anti-tubercular activity of garlic on multiple-drug re-
sistant Mycobacterium was investigated among Nigerian
HIV-infected-persons and it exhibited maximal activity
against all isolates even at reduced concentrations. Only

406 Int. J. Med. Med. Sci.
two of the standard anti-tubercular antibiotics used,
streptomycin and rifampicin, showed significant activity
against isolates tested (Dibua, 2010).
Antioxidant
Whole garlic and aged garlic extract exhibit direct
antioxidant effects and enhance the serum levels of two
antioxidant enzymes, catalase and glutathione
peroxidase (Prasad et al., 1995). Garlic extract, allicin is
efficiently scavenged exogenously generated hydroxyl
radicals in a dose dependent fashion, but their effective-
ness was reduced about 10% by heating to 100°C for 20
min. Other garlic constituents, such as S-allyl cysteine,
also confirmed significant antioxidant effects. The sulfur
compounds found in fresh garlic appear to be nearly
1000 times more potent as antioxidants than crude, aged
garlic extract. Garlic (both the homogenate of 10% in
physiological saline solution and its supernatant) was
able to reduce the radicals present in cigarette smoke
(Torok et al., 1994).
Drug toxicities and pharmacokinetics
Glutathione is a compound necessary for liver to facilitate
detoxification of substances. It has been hypothesized
that garlic organo-sulfur compounds may be able to
prevent glutathione depletion. Patients who experience
increasing in reactive oxygen induced stress on liver
function may be protected by garlic ingestion (Sabayan et
al., 2006). It was found in E. coli cultures that aged garlic
extract, S-allyl cysteine, diallyl sulfide and diallyl disulfide
do not interfere with the antibiotic activity of gentamycin
but may improve gentamycin-induced nephrotoxicity
(Maldonado et al., 2005). Aged garlic has also been
shown to reverse oxidant effects of nicotine toxicity in rat
studies. More researches are required in the future garlic
may be a unique choice to help minimize the toxic effects
of therapeutic drugs (Sener et al., 2005).
Reduces stress
Among the many uses of garlic, it appears to have the
fortunate capacity for protecting against the negative
effects of stress that affects the autonomic nervous and
neuroendocrine system. Rats that were trained with
endurance exercises to physical fatigue enjoyed improv-
ed parameters of aerobic glucose metabolism, attenuated
oxidative stress, and vasodilations, when given garlic at a
dosage of 2.86 g/kg for 30 min before exercise (Morihara
et al., 2006). In rats exposed to psychologically stressful
situations, aged garlic extracts significantly prevented the
decreases in spleen weight seen in control animals.
Additionally, the garlic significantly prevented the reduc-
tion of hemolytic plaque forming cells in spleen cells.
Moreover, garlic was able to block the lipopolysaccharide
induced immune cytokine and plasma corticosterone and
catecholamine changes following cold water immersion
stress (Nance et al., 2006). Aged garlic extract is also
effective to prevent adrenal hypertrophy, hyperglycemia
and elevation of corticosterone in hyperglycemic mice
induced by immobilization stress. Given the extreme
chronic stress many people now face in their daily life,
garlic may prove useful to counter the negative impact of
this stress on human physiology (Kasuga et al., 1999).
Adverse effects of garlic
The main adverse effect commonly associated with garlic
intake is breath odor, especially when raw forms of the
herb are used. Nausea and vomiting are other major
adverse effects and care should be taken in consuming
high quantities. Although an entire bulb produces little
juice, it is potent and can act as a strong emetic, even in
small quantities. Although garlic generally poses little in
terms of safety issues, there are isolated cases of topical
garlic burns (Friedman et al., 2006) and anaphylaxis (Yin
and Li, 2007). Rare garlic allergy has been attributed to
the protein allinase, which has induced immunoglobulin E
(IgE) mediated hypersensitivity responses from skin prick
testing (Kao et al., 2004). As a result, the literature has
generally cautioned against using garlic while using
anticoagulant therapy. There is a reported case of
spontaneous spinal or epidural hematoma in an 87 years
old man, with associated platelet dysfunction related to
excessive garlic ingestion (Saw et al., 2006).
CONCLUSION
Garlic, from crushed to capsules, is consumed through-
out the world. This review paper demonstrated some of
the benefits of garlic for its potential uses in preventing
and curing different diseases, and acting as antioxidant
for many radicals. Fresh and powdered garlic are popular
for food flavor and should continue to be used. Today,
with the ever-growing resistant organisms, taking of garlic
extract remains a powerful antimicrobial agent. Clearly
more studies are needed to refine the use and
improvement of the efficacy of this important medicinal
plant.
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