Lasers in Surgery and Medicine 39:297–301 (2007)
Hair Stimulation Following Laser and Intense Pulsed Light
Photo-Epilation: Review of 543 Cases and Ways to Manage It
Andrea Willey, MD,
* Jaioae Torrontegui, RN,
Jose Azpiazu, MD,
and Nerea Landa, MD
Oregon Health & Science University, Portland, Oregon 97239
Dermitek Clinic, Bilbao, Basque Country, Spain
Stimulation of terminal hair growth following photo-
epilation is a poorly understood problem with signiﬁcant
clinical relevance. We have observed this problem in a
number of patients in our dermatologic laser practice in
Basque Country Spain. To better evaluate the incidence
and character of this phenomenon a retrospective chart
review was performed on all patients who received laser
and intense pulsed light (IPL) photo-epilation at this single
center within the 5-year period from December 1998 to
December 2003. Patient images before and after treat-
ments were compared (digital images after 1999 and non-
digital images before year 1999) and the medical history
Five hundred forty-three patients with Fitzpatrick skin
types II, III, and IV (13%, 68%, and 19%, respectively)
ranging in age from 16 to 52 years received laser and/or IPL
hair photo-epilation of the beard, neck and chin areas,
excluding the upper lip. The number of treatments received
by each patient ranged from 3 to 23. Hair epilation
treatments were performed using a long pulsed 755 nm
alexandrite laser (Gentlelase, Candela, Wayland, MA) and
IPL source (Epilight, Lumenis, Santa Clara, CA) and a
1,064 nm Nd:YAG (Lyra, Laserscope, San Jose, CA). Only
10% of the treatments were performed with the 1,064 nm
Nd:YAG laser. Treatments were usually performed every
2– 3 months. The alexandrite laser was used in 85% of the
treatments, IPL in 10% and Nd:YAG in 5%. The parameters
used are listed in Table 1.
Of the 543 patients who received laser/IPL hair photo-
epilation, 57 (10.49%) demonstrated an increase in hair
growth compared to baseline. The increased hair growth
occurred within the area that was treated and also in
the areas bordering the treated area, and appeared thicker
and darker than the hairs initially treated (Figs. 1 and 2).
An additional 44 (8.10%) patients demonstrated no
apparent reduction in hair growth following treatment.
Four hundred twenty-four patients (78.08%) demonstrated
a decrease in hair growth with ongoing treatments.
Only 14 patients (2.5%) were discharged from the clinic
due to near complete hair reduction. These results are
summarized in Table 2.
The increased terminal hair growth occurred mostly in
areas in which ﬁne hair or both ﬁne and coarse hair was
present prior to initiation of treatment. Hair growth
occurred with greater frequency in patients treated with
the Alexandrite and IPL devices compared those treated
with the Nd:YAG, however, the later device was used less
frequently. Patients that developed terminal hair growth
were in the following age groups: 19–31 years, 44 patients;
30–40 years, 8 patients; greater than 40 years, 5 patients.
The onset of increased terminal hair growth was noted
between the third and tenth treatment in 39 (72.2%) of 57
patients, and 11 (19%) of 57 between the third and fourth
treatment. Most patients had a normal hormonal history.
Sixteen patients had irregular menses or documented
Because the terminal hair growth occurred both within
the treated areas and also at the periphery of treated areas
it was thought that sub-therapeutic thermal energy
delivered to nearby follicles induced terminal hair growth.
Subsequent application of cold packs surrounding the
treatment area during treatments and treating all
patients with two passes has minimized the incidence of
terminal hair growth (Fig. 3). Since we have instituted this
method in our clinic 2 years ago, we have treated over 200
patients and have not had any patients with hair growth
Despite the widespread use of lasers and IPL for hair
reduction, the biologic mechanism of photo-epilation is
largely unknown. Upon treatment with a laser or IPL
device, light is absorbed over millisecond pulse durations by
melanin contained within melanosomes in the hair matrix
and within keratinocytes in the hair shaft [1,2]. Heat
energy is transferred from the follicular matrix to the
surrounding non-pigmented follicular epithelium and
perifollicular dermis [1–4]. Sufﬁcient thermal injury to
the follicle and its surrounding tissue results in miniatur-
ization of follicles such that they become clinically unap-
parent for a variable duration of time . The precise target
of thermal injury from adjacent melanosomes or the
*Correspondence to: Andrea Willey, MD, Oregon Health &
Science University, Portland, OR. E-mail: firstname.lastname@example.org
Accepted 27 December 2006
Published online 25 April 2007 in Wiley InterScience
ß2007 Wiley-Liss, Inc.
subsequent biologic events that lead to clinical hair
reduction is not understood. Potential targets include cells
critical for follicular cycling, including the follicular stem
cells located within the bulge area of the outer root sheath
and cells of the follicular papilla . This may include cell
populations necessary for communication during follicular
cycling as well as cell populations essential for follicular
Histological studies have shown that within a photo-
epilated area not all of the follicles are thermally injured
TABLE 1. Laser Hair Epilation Devices and Parameters
(J) Spot size
Alexandrite gentle plus
755 12– 18 12 and 18 mm 3 Cryogen spray
) 645, 690, and 735 35– 38 4.5 cm 0.5 cm 3– 20 Cold gel
Long pulsed Nd:YAG (Lyra
) 1,064 24– 30 10 mm 35– 50 Cold gel
Fig. 1. A: Before photo-epilation. B: Terminal hair growth
stimulation within and around the photo-epilated area after
12 sessions. [Figure can be viewed in color online via
Fig. 2. A: Before photo-epilation. B: Terminal hair growth
stimulation within and around photo-epilated area after ﬁve
sessions. [Figure can be viewed in color online via www.
298 WILLEY ET AL.
[1,6], which suggests that some follicles are more suscep-
tible to photo-thermal injury than others. Differences in the
content of melanin associated with differing stages in the
hair cycle may account for this, since melanogenesis ceases
during catagen and telogen, commencing again during
anagen. Follicles in early anagen have been thought to be
most susceptible to photo-epilation since they contain
melanin which is located high in the dermis relative to late
anagen follicles residing deep in the subcutaneous tissue
beyond the optical penetration of red and near infrared
wavelengths [1,6]. Although the degree of melanin within
growing hairs is associated with efﬁcacy of treatment [1,2],
the degree to which melanin in the shaft versus the matrix
plays a role in affecting efﬁcacy depends upon whether the
target of thermal injury is primarily in the follicular stem
cells located in the bulge or the follicular papilla; heat
transferred from the shaft may heat primarily the follicular
infundibulum, whereas heat transferred from the matrix
may affect primarily the lower segment of the follicle.
Clearly, the efﬁcacy of laser hair epilation is ﬂuence
dependent, with the greatest amount of hair loss occurring
with higher ﬂuences [1,2]. Our observations that stimula-
tion of hair growth occurred at the periphery of treated
areas and that such hair growth could be minimized in the
same patients by applying cold packs to the surrounding
area and by using double passes with each treatment
suggests that sub-therapeutic ﬂuences at the periphery of
treated areas induce terminal differentiation of hair
growth rather than miniaturization. In other words,
instead of inducing miniaturization with a subsequent
prolonged telogen phase, follicles are instead shifted
towards terminal anagen hair growth.
Acquired localized hypertrichosis has been described in
various other settings of dermal injury, including terminal
hair growth at the periphery of a burn , transient limb
hypertrichosis associated with casting [8,9] peri-incisional
hypertrichosis following knee surgery or fracture sites
[10,11], distal hair growth following lymphadenectomy
, terminal hair growth at the site of bug bites 
and verruca vulgaris , local reactions to measles
, smallpox  and other vaccines , and chronic
rubbing, scratching, and biting associated with various
clinical situations [18,19]. Common to these widely varied
clinical presentations is the hypothesis that local hyper-
emia or inﬂammation may lead to localized terminal hair
Consonant with this idea, in addition to the complex
orchestrated events of follicular cycling that lead to
transformation of the follicle proper, the surrounding
follicular epithelium, associated dermal components, and
follicular vasculature undergo spectacular morphogenesis
with each growth cycle . A pronounced increase in
follicular vascularization occurs during anagen that is
accompanied by the upregulation of vascular endothelial
growth factor (VEGF) in outer root sheath keratinocytes
. This is followed by a rapid regression in perifollicular
vessels during catagen. Indeed growing follicles have much
higher perfusion requirements than resting follicles. These
accounts taken together with our observations raise the
compelling possibility that sub-therapeutic thermal injury
to the follicular vasculature may affect follicular cycling in
such a way to induce terminal hair growth rather than
miniaturization. Alternate hypotheses include the possibi-
lity that sub-therapeutic injury to the follicle may result in
the release of factors that alter follicular angiogenesis and
inﬂuence hair cycling. Additionally, both ultrastructural
and light microscopic studies have demonstrated the
uniform induction of perifollicular inﬂammation associated
with photo-epilation that persists for up to 2 weeks. Thus, it
follows that this local inﬂammatory response may also
affect follicular cycling in such a way to induce terminal
hair growth. While feasible, this idea does not explain why
some follicles react in this way and others do not, since
inﬂammation is not selective and thus not limited to less
thermally injured areas.
In our experience, there are ﬁve key factors associated
with failure to epilate and risk of hair stimulation:
(1) The thickness of treated hair: thicker hair is easier
to heat because the follicle reaches a high enough
temperature to destroy the cells critical for follicular
cycling; thinner hairs derived from follicles with less
chromophore absorb light energy less efﬁciently.
This explains the failure to epilate ﬁne hair that
occurred on areas on the face with ﬁne hair growth
Fig. 3. The use of cold packs to prevent peripheral stimulation
of hair growth. [Figure can be viewed in color online via
TABLE 2. Results
Subjects (%) Response
57 (10.49) Increased terminal hair growth
(equally distributed in all skin types)
44 (8.10) No change in hair growth
424 (70.08) Reduced hair growth (continued
14 (2.5) Persisting reduction in hair growth
(discharged from clinic)
LASER– INTENSE PULSED LIGHT PHOTO-EPILATION 299
and at other sites such as the abdomen, linea alba,
and back and shoulders in men.
(2) The color of treated hair: melanin within melano-
cytes of the follicular matrix and shaft is the
chromophore absorbed during photo-epilation, thus
darker hair is more efﬁciently heated.
(3) The depth of treated hair: optical penetration of
light may not be deep enough to adequately
thermally injure deeply growing anagen hairs in
some areas. This is only a hypothesis, however, since
we do not have in vivo real time sequences of lasers
reaching the target cells. It is possible that telogen
or late anagen follicles that are located in the dermis
may be more susceptible to hair growth stimulation.
Based on these observations, our current photo-epilation
protocol includes the use of ice packs for all patients with
ﬁne hair growth on facial or body areas. In addition, we
have observed in side to side studies that two passes with a
long pulsed 755 nm Alexandrite laser using an 18 mm spot
size is more effective than a single pass (unpublished work
by N.L.). Our current technique is to use 12–14 J/cm
followed 1 minute later by a second pass using 8 –10 J/cm
Therefore, the amount of energy delivered is important in
deﬁnitive destruction of the key cells responsible of normal
follicular cycling. Sub-optimal energies seem to stimulate
these cells and induce longer thicker hairs as a consequence
of accelerating the transition from vellus to terminal
follicles. This phenomenon by which heat induces hair
growth has been previously observed [9,10].
In our cases, the presence of ﬁne hair prior to treatment
appeared to be the most important factor for increased risk
of paradoxical terminal hair growth. In addition, some
areas appeared to be at higher risk: terminal hair growth
occurred most often in the low maxillary or ‘‘beard’’ area,
neck, lateral cheeks and chin areas in young women with
either skin types II, III, or IV. Patients with hair
stimulation were found in all skin types. Differences in
skin type likely matters only in that lower energies may be
used in patients with darker skin types, and thus it was
probably easier to deliver sub-optimal energies. In these
darker skinned patients, the use of double passes at lower
energies should be attempted. Alternatively, a single
treatment using one pass followed by a second single pass
treatment 1 week later may also be tried. Terminal hair
growth was mostly noted between third and fourth
treatment, but also occurred as late as after the tenth
treatment. The application of cold packs surrounding the
treatment area during treatments and use of double pass
technique appears to minimize the incidence of terminal
hair growth within and around photo-epilated areas.
Referral to an electrologist may be considered for patients
who are at high risk.
Previous descriptions of hair stimulation have occurred
with the use of various devices, including 694 nm Ruby,
755 nm Alexandrite, and 810 nm Diode lasers in addition to
IPL sources [21–26]. It is unclear if hair removal with the
1,064 nm Nd:YAG laser is less inclined to cause hair
stimulation or if it is simply used less often as is the case
in our practice. The more common occurrence of hair
stimulation on the lower face in females observed in this
study is also consistent with other reports, however reports
on the back of men have also been described. Females with
vellus hair on facial ‘‘beard area’’ should be anticipated a
chronic treatment with on multiple going sessions for
Although the majority of women in this study had no
history of hormonal abnormalities, the true hormonal
status cannot be certain from historical data. Thus,
hormonal abnormalities may or may not be directly
involved hair growth stimulation. Nevertheless, hirsute
individuals may be at an increased risk regardless of
hormonal status. It is our observation that the amount of
vellus facial hair may change with different hormonal
cycles or may be induced by laser epilation during different
treatment sessions. In our experience photo-epilation
should be performed more frequently and with higher
energies to optimize efﬁcacy in these patients.
Previous reports of paradoxical hair growth associated
with laser-IPL photoepilation suggest the incidence is
uncommon [21–26]. In addition, because the majority of
the reported cases occurred in individuals of Fitzpatrick
skin types III–V it has been felt that these patients are at
greatest risk. Our observations suggest that hair growth
stimulation following laser/IPL photo-epilation may be
more common than previously recognized and that indivi-
duals of Fitzpatrick skin type II are also susceptible.
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