Finasteride is a specific inhibitor of 5a-reductase, which is the enzyme responsible for converting testosterone into DHT (dihydrotestosterone). This drug can efficiently reduce the serum concentration of DHT, therefore minimizing the unwanted androgenic effects that result from its presence. The effect of this drug is quite rapid, suppressing serum DHT concentrations as much as 65% within 24 hours after taking a single 1 mg tablet. Medically, this drug has been marketed to treat two specific conditions. The first release of finasteride in the U.S. was under the brand name of Proscar, made for use by patients with benign prostate hyperplasia (prostate enlargement). More recently (December 1997), finasteride was approved for use as an anti-balding medication. We now have the additional brand name Propecia, which is the same drug but the tablet contains only 20% of the Proscar dosage. Scientists have long believed that DHT was the main culprit in many cases of male hair loss (along with genetic factors), so there was little doubt after the release of Proscar that finasteride would eventually be used for this purpose. It has provided what many feel is a breakthrough for men with hair-loss problems.
Due to the very specific nature of finasteride, it has little effect on the other hormones in the body. It has no affinity for the androgen receptor, and does not exhibit any androgenic, antiandrogenic, estrogenic or antiestrogenic properties. It should have no impact on circulating levels of cortisol, thyroid-stimulating hormone, or thyroxine, nor should it alter HDL/LDL cholesterol levels. Changes in luteinizing hormone (LH) or follicle-stimulating hormone (FSH) are also not notable, and it is not shown to have an effect on the hypothalamic-pituitary-testicular axis. In a small percentage of cases the decreased DHT level did produce symptoms of sexual disinterest/dysfunction. Although this is not a common complaint, this problem can usually be resolved quickly by discontinuing the drug. It is also interesting that finasteride has been shown to increase the circulating levels of testosterone by roughly 15%, since a greater amount of the androgen is being left unaltered by the reductase enzyme.