Finasteride side effects: what every patient should know

The side effect profile of finasteride has been extensively studied in clinical trials involving thousands of men.

According to the BNF, the following side effects are reported at both the 1 mg (hair loss) and 5 mg (prostate) doses.

Common side effects (affecting up to 1 in 10 men):

• Reduced libido (sex drive): reported by approximately 1 to 2 per cent of men in clinical trials
• Erectile dysfunction: affects around 1 to 2 per cent of users
• Reduced ejaculate volume: the amount of semen produced during ejaculation may decrease

Less common side effects (affecting up to 1 in 100 men):

• Breast tenderness or swelling (gynaecomastia): related to the hormonal changes caused by the medication
• Skin rash: mild allergic reactions have been reported
• Testicular pain: occasional discomfort has been documented

Why do these side effects occur?

Finasteride works by blocking the conversion of testosterone to dihydrotestosterone (DHT). DHT plays a role in sexual function, which is why reducing its levels can cause sexual side effects.

The 1 mg dose reduces DHT levels by approximately 65 per cent, while the 5 mg dose reduces them by around 70 per cent.

It is important to note that in clinical trials, sexual side effects also occurred in the placebo group, suggesting that awareness of potential side effects can influence their reporting.

Most men who experience these effects find that they resolve within a few weeks of starting treatment or after stopping the medication.

Sexual side effects are the most discussed aspect of finasteride treatment, and it is important to examine the evidence objectively.

Clinical trial data (1 mg for hair loss):

• In the pivotal trials, 3.8 per cent of men on finasteride reported any sexual side effect, compared to 2.1 per cent on placebo
• The absolute difference is therefore small, at approximately 1.7 per cent
• Most side effects resolved during continued treatment or after discontinuation

What the research shows:

• A systematic review published in the Journal of the American Academy of Dermatology confirmed that sexual side effects are uncommon and usually reversible
• The incidence appears similar at 1 mg and 5 mg doses, suggesting the effect is not strongly dose-dependent
• Younger men (under 40) may be more likely to report these effects, possibly due to greater awareness and concern

The nocebo effect:

• Several studies have demonstrated that men who are warned about sexual side effects before starting finasteride are significantly more likely to report them
• This does not mean the side effects are imaginary, but it highlights the role of expectation in symptom reporting
• Open, balanced counselling from your prescriber is important

Practical advice:

• If you experience sexual side effects, discuss them with your doctor rather than stopping the medication abruptly
• In many cases, the effects resolve with continued use
• If they persist, your doctor may suggest a dose reduction, a treatment break, or switching to an alternative hair loss treatment

A small number of men have reported persistent sexual, neurological, and psychological side effects after stopping finasteride.

This has been termed post-finasteride syndrome (PFS), though it remains a subject of scientific debate.

What the MHRA says:

The MHRA updated the product information for finasteride in 2023 to include warnings about:

• Persistent sexual dysfunction that may continue after treatment is stopped
• Depression and, in rare cases, suicidal thoughts
• Male breast cancer (very rare)

The current evidence:

• Large-scale studies have not conclusively established a causal link between finasteride and persistent sexual side effects
• However, case reports and patient registries document individuals who experienced lasting symptoms
• The mechanism by which this might occur is not fully understood, though theories include neurosteroid disruption and epigenetic changes

Important context:

• The vast majority of men who take finasteride do not experience persistent side effects
• The risk appears to be very low, though exact figures are difficult to determine
• If you experience depression, anxiety, or persistent sexual problems while taking finasteride, stop the medication and consult your doctor promptly

Regulatory position:

Both the MHRA and the European Medicines Agency (EMA) have concluded that the benefits of finasteride outweigh the risks for approved indications, but they recommend that patients are informed about the possibility of persistent effects before starting treatment.

The potential impact of finasteride on mental health has received increasing attention from regulators and researchers.

What has been reported:

• Depression: listed as an uncommon side effect in the BNF. The MHRA has highlighted this risk in safety communications
• Anxiety: some patients report increased anxiety during treatment
• Cognitive changes: a small number of patients describe difficulties with concentration or memory, sometimes referred to as "brain fog"
• Suicidal ideation: very rare, but the MHRA has added this to the product information as a precautionary measure

The evidence:

• A large observational study published in JAMA Dermatology found no statistically significant increase in depression or anxiety among finasteride users compared to non-users
• However, individual case reports and smaller studies have documented mood changes in some patients
• DHT and other neurosteroids play roles in brain function, which provides a theoretical basis for these effects

What you should do:

• Be aware of your baseline mood before starting finasteride
• Monitor for any changes in mood, motivation, or emotional wellbeing
• Tell someone close to you that you are starting the medication so they can alert you to changes you might not notice yourself
• If you experience depression, anxiety, or any thoughts of self-harm, stop finasteride and contact your GP immediately
• These symptoms typically resolve after stopping the medication, but some patients may need additional support

Your prescriber should ask about your mental health history before prescribing finasteride and check in at follow-up appointments.

If you experience side effects from finasteride, there are several approaches your doctor may consider.

For mild sexual side effects:

• Continue the medication and monitor. Many side effects resolve spontaneously within the first three to six months
• Discuss any concerns openly with your partner and your doctor
• Maintain a healthy lifestyle: regular exercise, adequate sleep, and stress management all support sexual function

If side effects persist:

• Your doctor may suggest reducing the dose. Some dermatologists prescribe finasteride on alternate days or three times per week for hair loss to reduce side effects while maintaining some benefit
• Switching to topical finasteride may be an option, as it delivers lower systemic levels of the drug
• Consider alternative treatments such as minoxidil (for hair loss) or tamsulosin (for BPH)

For mood changes:

• Stop finasteride and seek medical advice promptly
• Your GP can assess whether the mood changes are related to the medication or have another cause
• Referral to a mental health professional may be appropriate

Before starting finasteride:

• Have an open discussion with your prescriber about the benefits and risks
• Consider your baseline sexual function and mental health
• Make an informed decision based on balanced evidence, not fear
• Remember that the overall risk of side effects is low, and most men tolerate the medication well

The NHS information page provides additional guidance on monitoring during treatment.