Finasteride causes sexual side effects in roughly 2 to 4 percent more users than a placebo, with decreased libido, erectile dysfunction, and changes in ejaculation being the most commonly reported. Beyond sexual effects, the drug carries warnings for mood changes, including depression and suicidal thoughts. Most side effects resolve after stopping the medication, but a subset of users report symptoms that persist for months or longer.
How Finasteride Works in the Body
Finasteride blocks an enzyme that converts testosterone into a more potent hormone called DHT. DHT drives both hair loss and prostate growth, which is why finasteride is prescribed at 1 mg for male pattern baldness and 5 mg for enlarged prostate. By lowering DHT levels, finasteride slows hair thinning and shrinks prostate tissue. But DHT also plays roles in sexual function, mood regulation, and other body systems, which explains why the drug’s side effects reach beyond the scalp or prostate.
One important detail: both the 1 mg and 5 mg doses lower DHT by similar amounts. A study comparing the two doses in 895 men found that the rate of reported sexual side effects was essentially equivalent between them. This means taking the lower dose for hair loss doesn’t necessarily give you a milder side effect profile.
Sexual Side Effects
Sexual problems are the most frequently reported category of side effects. In clinical trials involving over 1,500 men taking 1 mg daily, 4.2 percent experienced sexual side effects compared to 2.2 percent on placebo. A separate set of trials with nearly 1,900 men found very similar numbers: 3.8 percent on finasteride versus 2.1 percent on placebo. The specific breakdown in those trials looked like this:
- Decreased libido: 1.8 to 1.9 percent on finasteride versus 1.3 percent on placebo
- Erectile dysfunction: 1.3 to 1.4 percent versus 0.7 to 0.9 percent on placebo
- Ejaculation changes: 1.0 to 1.2 percent versus 0.4 to 0.7 percent on placebo, including reduced ejaculate volume
Those numbers come from controlled trials at the 1 mg dose. At the 5 mg dose used for enlarged prostate, the reported rates climb higher in some studies. One two-year trial found erectile problems in 15.8 percent of finasteride users compared to 6.3 percent on placebo, and ejaculation disorders in 7.7 percent versus 1.7 percent. The higher rates may partly reflect the older age of prostate patients, who are already more prone to sexual difficulties.
Effects on Fertility and Semen
Some men taking finasteride worry about its impact on fertility. A study of young men taking 1 mg daily found no significant effects on sperm concentration, total sperm count, sperm motility, or sperm shape. However, the FDA label notes that male infertility and poor semen quality have been reported after the drug went to market, and that semen quality has normalized or improved in some men after they stopped taking it. If you’re actively trying to conceive and have concerns, this is worth discussing with your prescriber.
Depression and Mood Changes
Finasteride’s FDA label includes depression and suicidal thoughts in its list of post-marketing side effects. The European Medicines Agency reviewed 313 reports of suicidal ideation linked to finasteride, though it noted these cases occurred against an estimated 270 million patient-years of exposure, making the absolute risk very low. The agency added that the exact frequency “cannot be estimated from available data.”
Despite the uncertainty around precise numbers, the signal was strong enough for regulators to act. The UK’s drug safety agency issued a formal reminder about psychiatric side effects, and the EMA introduced measures to minimize the risk of suicidal thoughts. The FDA label now advises doctors to inform patients about these possibilities. Symptoms to watch for include persistent low mood, loss of interest in activities, increased anxiety, and any thoughts of self-harm.
What Happens After You Stop
For most men who experience side effects, symptoms resolve after discontinuation. But this isn’t universal. The UK drug safety agency reviewed reports of sexual side effects and found that in nearly half of those cases, the outcome was recorded as “not recovered” or “not resolved.” The FDA label specifically notes that sexual dysfunction, including decreased libido and erectile dysfunction, “continued after discontinuation of treatment” in some users.
There is no reliable way to predict who will recover quickly and who won’t. Some men report improvement within weeks of stopping, while others describe symptoms lasting months or years. This uncertainty is one of the more frustrating aspects of the drug’s risk profile, because the clinical trials that got finasteride approved were not designed to track long-term outcomes after patients stopped taking it.
Post-Finasteride Syndrome
A small number of former users report a cluster of persistent symptoms severe enough to be described as a distinct condition: post-finasteride syndrome, or PFS. The FDA held a patient listening session on the topic, acknowledging it as “a rare condition that develops in a small subset of individuals.” Onset can happen during treatment or in the months after stopping.
The reported symptoms span three broad categories. Sexual symptoms include complete loss of libido, genital numbness, and inability to reach orgasm. Neurological and cognitive symptoms include difficulty concentrating, memory problems, slurred speech, insomnia, tinnitus, and what patients describe as severe “brain fog,” including slowed thinking and difficulty following conversations. Physical symptoms include muscle wasting, joint pain, dry skin and eyes, and weight gain.
Diagnosis is difficult because standard medical tests tend to come back normal. One patient described in the FDA session had three MRIs, two nerve conduction studies, and a brain wave test, all of which showed no abnormalities. There are no established biomarkers, no validated diagnostic test, and no proven treatment. The condition remains poorly understood and is not universally recognized across the medical community, which can make getting help particularly challenging for those affected.
Breast Changes and Rare Risks
Finasteride can cause breast tenderness or swelling in some men, a condition called gynecomastia. This occurs because blocking DHT can shift the balance between testosterone and estrogen.
There has also been concern about a possible link to male breast cancer. Data from clinical trials of the 5 mg dose showed a rate of 7.8 cases per 100,000 patient-years among finasteride users compared to 3.8 per 100,000 patient-years among non-users. The difference was not statistically significant, but the UK’s drug safety agency flagged it as a potential risk worth monitoring. The absolute numbers remain extremely small either way.
Weighing the Decision
The majority of men who take finasteride do not experience noticeable side effects. In clinical trials, the difference between finasteride and placebo was only about 2 percentage points for sexual symptoms. But the possibility of persistent effects after stopping, even if rare, changes the calculation for some people. If you do notice sexual changes, mood shifts, or cognitive symptoms after starting finasteride, those effects are well-documented and worth taking seriously rather than dismissing.