The main cause of hair loss is DHT (dihydrotestosterone). Finasteride works by stopping the conversion of testosterone to DHT, reducing DHT levels by up to 75%.

The well-known Finasteride product is Propecia. In a study, over 80% of the balding individuals who used it experienced rejuvenated growth, and about 5% commented that their improvement was dramatic.

As well as increasing the number of hairs, Propecia increases the shaft diameter of existing hairs. Most of the improvement, taking Finasteride, will come in the first year of use. Since the main purpose of this drug is to prevent further hair loss, I usually suggest it as a first-line treatment for younger patients, for whom maintaining the hair they have is of prime importance.

The studies mentioned above did show that there are some limited side effects. A very small number (less than 2%) showed erectile dysfunction and decreased libido, although the same complaints were found in about 1% of patients taking placebos. These effects are completely reversible if the drug is discontinued, and over half of patients who continued the drug with these side effects saw them disappear.

Finasteride has been around for over 14 years, and has not shown any long-term side effects.

Women should not use Finasteride. The drug acts on testosterone and will not have any effect on women, even in women with high testosterone levels. It has also been shown to cause defects in the sexual organs of male infants, so it is advised that pregnant females not take, or even handle, this drug. If the male is taking Finasteride, Merck feels confident that the trace amounts found in the semen do not put the fetus at risk. I sometimes hear this concern from patients, and suggest they use condoms during the pregnancy to completely eliminate the risk of Finasteride entering the woman's bloodstream.