Introduction: Many patients attending out patient clinics with a set of nonspecific symptoms and signs that suggest the diagnosis asthenia, without organic cause to explain the symptoms. In order to determine which of was these patients confirmed the diagnosis of functional asthenia, was performed a prospective nation wide, study to diagnose and quantify the intensity of fatigue in these patients and measure the effectiveness of sulbutiamine 400 mg/day treatment short-term drug frontline. Results: 74,7% of patients were females with a mean age of 43,7 ± 12,5 years old and 25,3% were males with a mean age of 41,7 ± 13,5 years old. At baseline, the FSS score was 49,7 ± 7,3 points; at day 7 was 37,2 ± 8,8 points with a decrease of 25,2% and at day 15 was 28,0 ± 9,8 points, with a decrease of 43,7% respective to baseline, being statistically significant (p< 0,0001 and p< 0,0001 at day 7 and day 15, respectively). The percentage of response to treatment was 77,7% at day 15. Sulbutiamine was a very well tolerated treatment, there were reported mild adverse events in 132 patients (38,7%) at day 7 and in 115 patients (33,7%) at day 15. Conclusions: Sulbutiamine 400 mg/day is a secure treatment, it is well tolerated and effective in improving the asthenia symptoms, as demonstrated in this clinical trial by the significant decrease in the FSS mean score and the percentage of patients with asthenia at day 15 of treatment.