From the FDA Drug Label
In man, a single 5-mg oral dose of finasteride tablets produces a rapid reduction in serum DHT concentration, with the maximum effect observed 8 hours after the first dose. The suppression of DHT is maintained throughout the 24-hour dosing interval and with continued treatment Daily dosing of finasteride tablets at 5 mg/day for up to 4 years has been shown to reduce the serum DHT concentration by approximately 70%. The median circulating level of testosterone increased by approximately 10 to 20% but remained within the physiologic range In a separate study in healthy men treated with finasteride 1 mg per day (n=82) or placebo (n=69), mean circulating levels of testosterone and estradiol were increased by approximately 15% as compared to baseline, but these remained within the physiologic range
- Finasteride affects testosterone levels by increasing them, with the median circulating level of testosterone increasing by approximately 10 to 20% after daily dosing of finasteride at 5 mg/day for up to 4 years.
- The increase in testosterone levels remains within the physiologic range.
- Additionally, finasteride at a dose of 1 mg per day increased mean circulating levels of testosterone by approximately 15% as compared to baseline, also remaining within the physiologic range 1
From the Research
Finasteride primarily affects testosterone metabolism by inhibiting the enzyme 5-alpha reductase, which converts testosterone to dihydrotestosterone (DHT), leading to a slight increase in circulating testosterone levels in the body, typically by about 15%. This mechanism is supported by a study published in 2014 2, which found that finasteride significantly reduces DHT levels by approximately 70% in the bloodstream and up to 90% in the scalp and prostate tissues.
Key Points
- Finasteride inhibits the conversion of testosterone to DHT, resulting in a decrease in DHT levels and an increase in testosterone levels
- The medication works by preventing testosterone from being converted to its more potent metabolite DHT, rather than by reducing overall testosterone production
- This selective action explains why finasteride can help with hair loss and prostate enlargement while generally maintaining normal sexual function in most men
- Some individuals may experience sexual side effects due to the hormonal changes caused by finasteride, as noted in a study published in 2009 3
Effects on Testosterone Levels
The effects of finasteride on testosterone levels are supported by a study published in 1992 4, which found that finasteride causes a profound decrease in prostatic DHT levels, with a reciprocal increase in testosterone levels. Another study published in 2018 5 found that finasteride inhibits the brain dopaminergic system and open-field behaviors in adolescent male rats, suggesting that finasteride may have effects on the brain and behavior.
Clinical Implications
The clinical implications of finasteride's effects on testosterone levels are significant, as noted in a study published in 1993 6. Finasteride is effective in treating conditions driven by DHT, such as male pattern baldness and benign prostatic hyperplasia. The standard dosage is 1mg daily for hair loss and 5mg daily for prostate conditions, with effects typically becoming noticeable after 3-6 months of consistent use. Overall, finasteride's effects on testosterone metabolism make it a useful medication for treating a range of conditions, while generally maintaining normal sexual function in most men.