Finasteride has a reputation problem that is partially deserved and partially the result of how the internet amplifies bad outcomes relative to neutral ones. Tens of thousands of men take it without significant problems. A smaller but real number experience sexual side effects. An even smaller number report persistent side effects after stopping. All of these things are true simultaneously.
Getting an accurate picture requires going back to the actual trial data rather than patient forum aggregations.
What the PLESS Trial Found
The Proscar Long-Term Efficacy and Safety Study (PLESS) is the most cited large trial on finasteride side effects. It followed 3,040 men over four years on either 5 mg finasteride or placebo, originally studying benign prostatic hyperplasia rather than hair loss (the hair loss dose is 1 mg, but the side effect mechanism is the same).
The published results showed:
- Erectile dysfunction: 8.1% in the finasteride group vs. 3.7% in placebo
- Decreased libido: 6.4% vs. 3.4%
- Ejaculation disorders: 0.8% vs. 0.1%
These are four-year cumulative rates. In the first year of the trial, the rate differences were more pronounced. In years two through four, the rate of new sexual side effects in the finasteride group declined and was no longer statistically different from placebo.
In the Merck clinical trials specifically for 1 mg finasteride (Propecia, the hair loss dose), conducted in men without prostate conditions, the reported rates were lower:
- Erectile dysfunction: 1.3% finasteride vs. 0.7% placebo
- Decreased libido: 1.8% finasteride vs. 1.3% placebo
- Ejaculation volume decrease: 1.2% finasteride vs. 0.4% placebo
The difference between PLESS and the Propecia trials reflects the dose (5 mg vs. 1 mg), the patient population (older men with prostate conditions vs. younger men with hair loss), and the duration. For men taking 1 mg for hair loss, the 1-2% range for sexual side effects from Merck’s own trials is the most directly applicable data.
The Placebo Effect Is Real
One notable feature of the finasteride side effect literature is that placebo groups consistently report sexual side effects too, at lower but still meaningful rates. This is the nocebo effect: knowing that a drug might cause sexual side effects increases the likelihood of reporting them.
A 2021 study published in the Journal of Sexual Medicine explicitly tested this. Researchers told one group of men about finasteride’s sexual side effects before prescribing it. Another group was given the medication without being informed of the specific side effects. The informed group reported sexual side effects at more than double the rate of the uninformed group.
This does not mean side effects are imaginary. It means the reported rate in trials includes a psychogenic component that is difficult to separate from pharmacological effect. The true pharmacological rate of sexual side effects is probably somewhere between the placebo rate and the finasteride rate in trials.
Post-Finasteride Syndrome
Post-finasteride syndrome (PFS) refers to a cluster of persistent side effects, primarily sexual dysfunction, depression, and cognitive symptoms, that some men report continuing after they stop taking finasteride. The Post-Finasteride Syndrome Foundation has documented case reports and advocates for research.
The FDA added a warning about persistent sexual side effects to finasteride’s label in 2012. The FDA label acknowledges that reports of sexual side effects persisting after discontinuation have been received.
What is documented: a genuine population of men who stopped finasteride and continue to experience sexual dysfunction, depression, and cognitive complaints that they attribute to the drug.
What is disputed: the prevalence, the mechanism, and whether the association is causal. PFS does not have an accepted biological mechanism. The limited studies that exist on PFS patients show some hormonal and neurosteroid abnormalities, but the sample sizes are small and the studies are not controlled. Epidemiological data on the true rate of persistent side effects after stopping finasteride does not exist in the way it does for, say, SSRI discontinuation syndrome.
A 2012 study in the Journal of Sexual Medicine found that 96% of men who stopped finasteride due to sexual side effects reported resolution within three months. This is the figure most frequently cited for reversibility. It comes from a study with significant limitations: it was not placebo-controlled, relied on self-report, and did not specifically recruit men reporting persistent problems. It cannot be used to dismiss persistent effects.
The honest position is that most men who experience side effects on finasteride see them resolve when they stop, but a smaller number do not, and the scientific understanding of why is incomplete.
Who Should Not Take Finasteride
Men who are planning to have children in the near term should discuss timing with a doctor. Finasteride reduces semen volume and can affect sperm production. Though semen levels should return to normal after stopping, the timing is unpredictable.
Men with a history of depression should discuss the mental health component of finasteride’s side effect profile with their provider before starting. The association between finasteride and depressive symptoms, while not established as causal, is worth understanding given the subjective nature of mood reporting.
Women who are pregnant or may become pregnant should not handle crushed or broken finasteride tablets. The drug can be absorbed through the skin and is teratogenic to a developing male fetus.
The Risk-Benefit Framing
For a man at Stage 3 male pattern baldness, finasteride has a well-documented 80-90% rate of halting or improving hair loss in clinical trials. The rate of sexual side effects at the 1 mg hair loss dose is approximately 1-3% above baseline. The rate of persistent side effects after stopping is uncertain but real.
That is the actual trade-off. Whether it is favorable depends on how much the hair loss matters, how old you are, your reproductive plans, and your individual risk tolerance.
Finasteride is not uniquely dangerous among medications. It is also not free of real risks. The clinical decision is straightforward only when the data is presented accurately.
For more on hair loss treatment at different stages, see The 7 Stages of Male Pattern Baldness Explained. For the minoxidil side of the equation, see How Long Does Minoxidil Take to Work?
