- Finasteride inhibits an enzyme called 5-alpha reductase, reducing levels of the androgen dihydrotestosterone (DHT). DHT plays a crucial role in male sexual development.
- The official drug label refers to reports of sexual dysfunction that continued after stopping the drug. In some men, these changes appear to be irreversible.
- Every year FDA receives reports of men who took finasteride and experienced problems with sexual function, changes to the penis and testicles, mental disturbances, insomnia and other adverse events.
- Feeling torn about whether to take finasteride? See Weighing the risks.
Mechanism of finasteride
Finasteride inhibits the enzyme 5-alpha reductase, which converts testosterone to another androgen called dihydrotestosterone (DHT). People born with a deficiency of the 5-AR enzyme appear to be girls at birth but develop male characteristics at puberty. Because they have small prostates and their hairline does not recede in adulthood, Merck sought to develop a drug that would induce a 5-AR deficiency in adult men. Learn more about the origins of finasteride
After Propecia was approved in 1997, the FDA required several updates to the label. In 2012, the label was revised to include reports of sexual adverse effects “that continued after stopping the medication”. Here are adverse effects from the current label, with underlining added for emphasis:
Comparing finasteride and dutasteride
Finasteride and dutasteride both inhibit the 5-alpha reductase enzyme, but there are a number of differences in their pharmacology, history and current use. Key information for both drugs is summarized in the following table.
|Brand name||Propecia (finasteride 1 mg) for AGA|
Proscar (finasteride 5 mg) for BPH
|Avodart (dutasteride 0.5 mg)|
|Drug developer||Merck & Co.||GlaxoSmithKline|
|Current drug marketer||Organon ||GSK |
|Regulatory status||Proscar approved in U.S. in 1992|
Propecia approved in U.S. in 1997
Prescribed off-label for prevention of prostate cancer
|Avodart for BPH approved in U.S. in 2001|
Approved for treatment of AGA in South Korea & Japan. Not approved for AGA in U.S. or Europe.
|Pharmacology & pharmacokinetics||Inhibits 5-AR type 2; weak inhibitor of 5-AR type 1|
Half life: 5-6 hours
|Inhibits 5-AR types 1 and 2|
Half-life: approx. 5 weeks
Notes: 1. Propecia and Proscar were transferred to Organon in a 2021 spinoff from Merck.
2. GlaxoSmithKline was renamed to GSK in 2022.
Some natural substances such as saw palmetto, black pepper and ginseng also have 5-alpha reductase inhibiting properties (see this bibliography).
Adverse events reported to FDA
This table summarizes adverse events of men 18–40 taking finasteride for hair loss, reported to FDA between 2018 and 2020. To minimize confounding factors, men taking finasteride for other reasons and men taking other medications or products were excluded.
Read more about adverse events and download the data here.
|Adverse event category||% of top 25 adverse events|
Includes Erectile Dysfunction, Loss of Libido & Sexual Dysfunction
Includes Depression, Anxiety & Suicidal Ideation
|Sexual anatomy & physiology|
Includes Testicular Pain, Genital Hypoaesthesia (penile numbness) & Penile Size Reduced
|Memory, cognition & attention|
Includes Amnesia, Cognitive Disorder & Disturbance in Attention
|Energy & musculoskeletal system|
Fatigue & Muscle Atrophy
Tinnitus & Vision Blurred
Prescrire International: Finasteride is ‘more dangerous than beneficial’
Since 1981, the French organization Prescrire International has published recommendations on drug safety and efficacy. In 2021, Prescrire added finasteride for hair loss to its Drugs to Avoid – 2021 update. Drugs in this list are “identified as more dangerous than beneficial” for their indications. The short paragraph on finasteride states, in part:
Finasteride 1 mg, a 5-alpha reductase inhibitor, has very modest efficacy against male-pattern baldness in men, slightly increasing hair density on the crown of the head (by about 10%), but only while treatment continues. Notable adverse effects include sexual dysfunction (erectile dysfunction, ejaculatory disorders, decreased libido), depression, suicidal ideation and breast cancer.Prescrire International. Drugs to Avoid 2021.
If you are considering taking finasteride, see Weighing the risks of finasteride.
Some men have reported symptoms remaining for years after stopping finasteride, which they attribute to having taken finasteride. To learn about these reports, see Persistent adverse effects.
This site has collected reports from men who took finasteride and got adverse effects. See Firsthand reports.
To learn the origins, regulatory history and more about finasteride, see the timeline.