About finasteride for hair loss

Key points

  • Finasteride induces a deficiency of the androgen dihydrotestosterone (DHT) by inhibiting the enzyme 5-alpha reductase (5-AR). DHT plays an important role in male sexual development. People born with a deficiency of this enzyme appear to be female at birth but mature into males around age 12. Learn more
  • 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, psychological adverse events, cognitive problems, insomnia and other adverse events.
  • Feeling torn about whether to take finasteride? See Weighing the risks.

Mechanism of finasteride

Finasteride inhibits an enzyme that converts testosterone to a more potent androgen called dihydrotestosterone (DHT). For more details on the drug mechanism, see PubChem, Propecia Prescribing Information (section 12) and Finasteride in StatPearls.

The drug was developed based on research on an intersex population in the Dominican Republic, first published in 1974. Certain individuals were raised as girls until, at approximately age 12, they grew penises and typically developed into males. They were found to have a deficiency in the enzyme 5-alpha reductase. They had smaller prostates and no recession of their hairline in adulthood. To learn more, see: How was finasteride invented?

FDA-approved label

Patient information is here, while Highlights of Prescribing Information contains more detailed information about adverse reactions and pharmacology.

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:

Patient information: PROPECIA as of 20 Jun 2021

In June 2021, FDA approved the addition of an adverse event, hematospermia (blood in semen), to the Propecia Prescribing Information.

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
Sexual function
Includes Erectile Dysfunction, Loss of Libido & Sexual Dysfunction
Psychological dimension
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.

More information

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.