- Mechanism of finasteride
- Adverse effects on drug label
- Commonly reported adverse events
- Comparing finasteride and dutasteride
- Advisory from Prescrire International
- More information
Mechanism of finasteride
Finasteride inhibits an enzyme which converts testosterone to another androgen called dihydrotestosterone (DHT). DHT plays a crucial role in male sexual development.
The enzyme finasteride inhibits is called 5-alpha reductase (5-AR). People born with a deficiency of this enzyme may 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 deficiency of the 5-AR enzyme in adult men. Learn more about the origins of finasteride
For details on the drug mechanism, see:
- Propecia Prescribing Information (section 12)
- StatPearls articles: Finasteride and 5 Alpha Reductase Inhibitors
- Aggarwal et al., 2010: An overview on 5α-reductase inhibitors
- PubChem: Finasteride
- On 5-alpha reductase and dihydrotestosterone
- Bibliography: 5-alpha reductase
- Bibliography: Dihydrotestosterone
- Protein Data Bank: structure of 5-alpha reductase 2
- NCBI Gene: SRD5A2
Adverse effects on drug label
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:
Learn more and download documents: finasteride label • dutasteride label
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 (there are slight differences from the table below because the time frame is 2019–2021 rather than 2018–2020).
|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
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 types 2 and 3 ; weak inhibitor of 5-AR type 1|
Half life: 5-6 hours
|Inhibits 5-AR types 1, 2 and 3 |
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. 3. After both drugs were approved, 5-AR type 3 was identified, and it was reported to be “potently inhibited” by finasteride and dutasteride (Yamana et al., 2010)
Some natural substances such as saw palmetto, black pepper and ginseng also have 5-alpha reductase inhibiting properties (see this bibliography).
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 Lasting dysfunctions after use of finasteride or dutasteride.
This site has collected reports from men who took finasteride and experienced adverse effects. See Firsthand reports.
To learn the origins, regulatory history and more about finasteride, see the timeline.