Finasteride is a 5-alpha reductase inhibitor. This prevents steroids that can be converted to DHT from doing so. As discussed earlier, DHT causes baldness, prostate enlargement and under normal circumstances does not contribute to skeletal muscle growth. If a steroid is converted to DHT at a rate of 20%, that means there is 20% less steroid working to increase muscle growth. Therefore, finasteride is very useful not only in decreasing hair loss and prostate growth, but may also increase the effectiveness of a stack that contains steroids that convert to DHT. Most AAS users believe that adding finasteride will decrease the effectiveness of the stack. In fact, several papers have shown that finasteride does not reduce the anabolic effect of testosterone when co-administered to animals1,2.Since DHT is deactivated in skeletal muscle beforeit can act, it makes sense that inhibiting conversion to DHT would not reduce anabolism. Reducing conversion to DHT will reduce the increase in aggressiveness and some of the strength gains of a cycle but the effect on anabolism should be minimal. As discussed previously, DHT is partly responsible for feedback inhibition of natural testosterone production. The use of finasteride can help to reduce shutdown and to increase testosterone levels3. Reducing DHT levels too low can result in a loss of libido and may result in gynecomastia from offsetting the estrogen/androgen ratio.
1. Borst SE, Conover CF, Carter CS, Gregory CM, Marzetti E, Leeuwenburgh C, Vandenborne K, Wronski TJ. Anabolic effects of testosterone are preserved during inhibition of 5alpha-reductase. Am J Physiol Endocrinol Metab. 293(2):E507-14, 2007
2. Borst SE, Lee JH, Conover CF. Inhibition of 5alpha-reductase blocks prostate effects of testosterone without blocking anabolic effects. Am J Physiol Endocrinol Metab. 288(1):E222-7, 2005
3. Steiner JF: Clinical pharmacokinetics and pharmacodynamics of finasteride. Clin Pharmacokinet. Jan;30(1):16-27, 1996