Finasteride is a medication primarily prescribed for male pattern baldness (androgenetic alopecia) and benign prostatic hyperplasia (BPH), an enlarged prostate. Approved by the FDA for BPH in 1992 and male pattern hair loss in 1997, it targets a specific hormonal pathway. The medication works to reduce levels of dihydrotestosterone (DHT) in the body. Understanding how finasteride interacts with the body, its processing, and the effects of stopping treatment can provide clarity for individuals considering or currently using this medication.
How Finasteride Affects the Body
Finasteride is a 5-alpha reductase inhibitor, specifically targeting the Type II isoenzyme. This enzyme converts testosterone, a male hormone, into dihydrotestosterone (DHT). DHT is a more potent androgen playing a significant role in male pattern baldness and prostate enlargement.
By inhibiting 5-alpha reductase, finasteride lowers DHT levels in various tissues, including the scalp and prostate gland. For male pattern baldness, this DHT reduction helps slow or prevent further hair loss by reducing follicle miniaturization and promoting hair growth. For BPH, lower DHT levels contribute to a reduction in prostate volume, which can alleviate urinary symptoms associated with an enlarged gland. Finasteride can reduce serum DHT levels by about 70%.
How the Body Processes Finasteride
Finasteride is absorbed after oral administration, with peak blood levels typically reached within one to two hours. The concept of half-life describes the time it takes for the concentration of a drug in the body to decrease by half. For finasteride, the elimination half-life from the bloodstream is relatively short, generally 5 to 6 hours in younger men (18-60 years). In men over 70, this half-life can be slightly prolonged to approximately 8 hours.
Finasteride undergoes extensive metabolism primarily in the liver, involving the cytochrome P450 enzyme system (CYP3A4). This process converts finasteride into inactive or significantly less active metabolites. Once metabolized, the drug and its byproducts are eliminated through both urine and feces. Approximately 57% to 64% of the dose is excreted in the feces, while 32% to 39% is eliminated via urine.
While the physical drug may be cleared relatively quickly, its biological effects on DHT levels can persist for a longer duration. This is because finasteride binds to the 5-alpha reductase enzyme, and its inhibitory action on DHT production continues even as the drug concentration in the bloodstream declines. Liver function is important, as the liver is the primary site of metabolism, though no specific dose adjustments are typically needed for kidney impairment due to compensatory fecal excretion.
What Happens When Finasteride is Stopped
Upon discontinuing finasteride, the body’s ability to produce DHT is no longer inhibited, leading to a gradual return of DHT levels to pre-treatment concentrations. DHT levels typically normalize within approximately 14 days after the last dose. Consequently, the processes finasteride counteracted will resume.
For male pattern hair loss, the process usually restarts within several months to a year after stopping. Any hair gained or maintained will likely be lost, and the previous balding pattern will re-establish. The ongoing suppression of DHT is necessary to sustain the benefits of finasteride for hair.
For most men, side effects experienced while taking finasteride, such as sexual issues, tend to resolve within weeks to a few months after discontinuation. However, a small percentage may report persistent symptoms, sometimes referred to as Post-Finasteride Syndrome. While rare, these lingering effects can include sexual, physical, or neuropsychiatric symptoms. The resolution of side effects varies among individuals based on factors like overall health and dosage taken.