Fluoxetine

Key facts

• Fluoxetine (sold as Prozac and as generics) is an SSRI used for major depressive disorder, obsessive-compulsive disorder, bulimia nervosa, panic disorder and premenstrual dysphoric disorder.
• You take it once daily, usually in the morning; it can take 4 to 6 weeks to reach its full effect.
• Fluoxetine has a much longer half-life than other SSRIs. It and its active metabolite can stay in your body for several weeks after your last dose, which matters if you switch to another antidepressant or an MAOI.
• In people under 25, fluoxetine can increase suicidal thoughts and behaviour, especially in the first weeks of treatment. Seek urgent care for high fever, agitation or muscle rigidity, which can signal serotonin syndrome.

What fluoxetine treats

Fluoxetine treats major depressive disorder, obsessive-compulsive disorder (OCD), bulimia nervosa (binge eating and purging), panic disorder and premenstrual dysphoric disorder. In children and adolescents it is also used for depression and OCD under specialist supervision.

How fluoxetine works

Nerve cells release serotonin, a chemical messenger involved in mood and appetite, then reabsorb it. Fluoxetine blocks that reabsorption (reuptake), so more serotonin stays active between nerve cells. Unlike most SSRIs, fluoxetine and its main breakdown product are cleared very slowly, so its effects, and any interactions, can persist for weeks after you stop taking it.

Before you take it

• Never take fluoxetine with, or within 5 weeks of stopping, a monoamine oxidase inhibitor (MAOI); the combination can cause serotonin syndrome, which can be fatal. Because fluoxetine clears so slowly, this wait is longer than for other SSRIs.
• Tell your prescriber if you have bipolar disorder, a seizure history, liver disease or take blood thinners, other serotonergic drugs or NSAIDs, which raise bleeding risk when combined with fluoxetine.
• Do not stop fluoxetine suddenly without medical advice. Its long half-life means withdrawal effects, if they occur, tend to appear later and unfold more gradually than with other antidepressants.
• If you are pregnant, planning pregnancy or breastfeeding, discuss risks and benefits with your prescriber before starting or stopping.

Side effects

Common effects, often easing after the first few weeks: nausea, headache, insomnia, dry mouth, sweating and reduced sex drive.

Stop and seek urgent medical care for any of these:

• New or worsening suicidal thoughts, especially if you are under 25.
• Agitation, high fever, fast heartbeat, muscle twitching or confusion (serotonin syndrome).
• Unusual bleeding or bruising.
• Severe rash, swelling or difficulty breathing.

Safety essentials

• Fluoxetine's long half-life means it stays in your system for weeks after your last dose. Wait at least 5 weeks after stopping fluoxetine before starting an MAOI, and never combine it with one while taking it.
• This class carries a warning for increased suicidal thinking in people under 25, particularly in the first weeks of treatment. Watch for agitation or talk of self-harm.
• Tell every prescriber about all serotonergic medicines you take (triptans, tramadol, other antidepressants, St John's wort) to avoid serotonin syndrome, and taper off under medical guidance rather than stopping abruptly.

This page is educational and does not replace advice from a doctor or pharmacist who knows your health history.