DHT Blocker Side Effects: What the Finasteride, Dutasteride and Saw Palmetto Trials Report

The documented side effects of DHT blockers are sexual: lower libido, erection problems and ejaculation problems. In the hair loss trials, most men did not report them.

I am against these drugs and I have said so many times. That is exactly why I want to give you the real numbers, including the ones that do not help my case.

What is a DHT blocker?

• Finasteride. A prescription drug that inhibits the type II form of 5-alpha reductase, the enzyme that turns testosterone into DHT. The hair loss dose in the trials is 1 mg a day.
• Dutasteride. A prescription drug that inhibits both type 1 and type 2 of the enzyme. The trials below used 0.5 mg a day.
• Saw palmetto. A plant extract with antiandrogenic properties, sold as a supplement. The hair studies used 100 to 320 mg.

DHT vs testosterone

DHT is made from testosterone. It binds the androgen receptor more strongly, and unlike testosterone it cannot be turned into estrogen. A DHT blocker does not lower testosterone. It stops the conversion. What DHT does for blood sugar, muscle and stress is in How to increase DHT.

DHT and hair loss

The standard position is that male pattern hair loss is an androgen-dependent shrinking of scalp follicles, with scalp DHT as a contributing cause. That is the premise of every trial below.

My view is different. I think DHT in the scalp is a symptom of a metabolic problem and not its root. That is my reading, and the trials were not designed to test it.

Do DHT blockers work?

For hair counts, yes. I will not pretend otherwise.

• Finasteride. In two one-year trials in 1,553 men aged 18 to 41, finasteride added 107 hairs over placebo in a one-inch circle of balding scalp at one year, and 138 at two years. The starting count was 876. Men on placebo kept losing hair. (Kaufman 1998)
• Five years. The extension of those trials reported durable improvement over five years against progressive loss on placebo. (Finasteride Study Group 2002)
• Dutasteride. In 917 men over 24 weeks, 0.5 mg of dutasteride raised hair count and width more than finasteride did. (Gubelin Harcha 2014)
• Saw palmetto. A systematic review found 5 randomized trials and 2 cohort studies with positive results, including a 27% improvement in total hair count. The authors say robust high-quality data are lacking. (Evron 2020)

DHT blocker side effects: what the trials report

• The pooled risk. A meta-analysis of 15 placebo-controlled hair loss trials with 4,495 subjects found a 1.57-fold risk of sexual dysfunction on these drugs. For finasteride it was 1.66. For dutasteride it was 1.37, which was not statistically significant. (Lee 2019)
• A second meta-analysis disagrees in part. Across 17 trials and 17,494 patients, the risk of sexual dysfunction was 2.56 times higher in men treated for the prostate. In men treated for hair loss it was 1.21 and not statistically significant. (Liu 2016)
• The original finasteride trials summarized adverse effects as minimal.
• A large clinic series. Of 3,177 Japanese men on 1 mg, adverse reactions were recorded in 0.7%, and seven men stopped. The authors note that many patients did not come back for follow-up. (Sato 2012)
• Dutasteride. In the 917-man trial the number and severity of adverse events were similar across the groups, placebo included.
• Saw palmetto. A safety review of 40 articles found adverse events mild and similar to placebo. The most frequent were abdominal pain, diarrhoea, nausea, fatigue, headache, decreased libido and rhinitis. (Agbabiaka 2009)

The honest part: most men in the trials report nothing

A relative risk of 1.57 sounds large. But it is a multiple of a low starting rate. Even at the 5 mg prostate dose, the published trials put sexual side effects at 15% of men at most after a year. The hair dose is 1 mg.

Expectation is part of it. 107 men with an enlarged prostate took finasteride 5 mg for a year. Half were told it could cause sexual side effects. Half were not. 43.6% of the informed group reported one or more sexual side effects, against 15.3% of the others. Same drug, same dose. (Mondaini 2007)

So some of what men feel on finasteride comes from knowing what to look for. Posts like mine add to that. You should know it.

What the trials miss

• Symptoms that stay. 71 otherwise healthy men aged 21 to 46 reported sexual side effects that lasted at least 3 months after stopping finasteride. 94% had low libido and 92% erectile dysfunction. The average was 40 months after stopping. The authors list selection bias and recall bias as limits, and the study cannot say how often this happens. (Irwig 2011)
• Mood. In 93,197 Canadian men aged 66 and older who started one of these drugs, suicide was not increased. Self-harm was 1.88 times higher and depression 1.94 times higher in the first 18 months. In absolute terms that was 17 and 237 extra events per 100,000 patient-years. (Welk 2017)
• Young men. In the World Health Organization's side effect database, reports of suicidality were disproportionately frequent for finasteride in patients under 45 who took it for hair loss. The authors say this may reflect stimulated reporting after 2012. (Nguyen 2021)
• Blood sugar. In British men treated for the prostate, new type 2 diabetes was more frequent on dutasteride and finasteride than on tamsulosin, with hazard ratios of 1.32 and 1.26. (Wei 2019)

The hair trials ran for one to two years in selected men and counted what men volunteered. Rare, lasting or slow problems are exactly what that design does not see.

What I think

DHT blockers are basically free. They can cost you libido, mood and metabolic rate. For most men they will not. For some they will, and nobody can tell you in advance which group you are in. I would not take that bet for hair.

That is my opinion. It is not advice to stop a drug you were prescribed. If you are on finasteride or dutasteride and want off, talk to the doctor who prescribed it.

What I would try first is in the hair loss guide.

Quick answers

What are the side effects of DHT blockers?

The documented ones are sexual: lower libido, erectile dysfunction and ejaculation problems. A meta-analysis of 15 hair loss trials found a 1.57-fold risk of sexual dysfunction on finasteride or dutasteride compared with placebo.

Do most men get side effects from finasteride?

No. In the trials most men did not report any. A Japanese clinic series of 3,177 men recorded adverse reactions in 0.7%, though many patients were not followed up.

Do DHT blockers work?

For hair counts, yes. In trials of 1,553 men, finasteride 1 mg added 107 hairs over placebo in a one-inch scalp circle at one year and 138 at two years, while men on placebo kept losing hair.

What is the difference between DHT and testosterone?

DHT is made from testosterone by the enzyme 5-alpha reductase. It binds the androgen receptor more strongly and cannot be converted into estrogen. DHT blockers inhibit that enzyme.

Does saw palmetto have side effects?

A review of 40 articles found them mild and similar to placebo: abdominal pain, diarrhoea, nausea, fatigue, headache, decreased libido and rhinitis.

Can finasteride side effects be permanent?

Some men report symptoms that persist after stopping. A study interviewed 71 such men, on average 40 months after they quit. It could not measure how common this is.