The MHRA Drug Safety Update of 29 April 2024 ("Finasteride: reminder of the risk of psychiatric side effects and of sexual side effects (which may persist after discontinuation of treatment)") explicitly lists psychiatric reactions among the risks patients must be counselled on. The MHRA further strengthened these warnings in a Drug Safety Update on 11 May 2026, and added a precautionary warning for dutasteride (data for dutasteride were very limited). Patient alert cards were introduced into finasteride packs as part of these updates.
"Brain fog" is not a specific medical diagnosis but a cluster of symptoms patients describe: difficulty concentrating, slowed thinking, word-finding problems, mental fatigue. On finasteride, these symptoms have been reported by a minority of users and feature in the psychiatric reactions the MHRA flagged in 2024. Mechanistically, finasteride blocks 5-alpha-reductase, which also has roles in the brain (neurosteroid synthesis); whether this explains the symptom in every reporter is not settled. The lived experience is real; the precise biology is still being studied.
How long does it last
In most cases reported in the regulator's case series, symptoms resolved within weeks of stopping the medication. The MHRA explicitly recognises that in a small number of cases psychiatric symptoms have been reported to persist after discontinuation. There is no formula for predicting which patients will experience persistence.
What to do if it happens to you
Contact your prescriber promptly. Document the symptoms (onset, severity, timing, what makes it better or worse). Your clinician will discuss whether to continue at the current dose, reduce, switch (e.g. to topical only) or stop. Reporting via the MHRA Yellow Card scheme contributes to the regulator's ongoing review.
When to seek urgent care
If brain fog is accompanied by suicidal thoughts, severe mood changes, or any symptom you find frightening, contact Samaritans on 116 123 (free, 24/7), NHS 111, or attend your nearest A&E immediately.
If you are experiencing this on treatment, it should be reviewed rather than guessed at. Our clinicians assess symptoms against your full history and adjust, switch or stop treatment as appropriate. If you are on medication from another provider, we can still review you.
If you are having thoughts of harming yourself, do not wait for a review. Contact your GP, call 111, or call the Samaritans free on 116 123 at any time. In an emergency call 999.
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