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Side effects of fluoxetine

Like all medicines, fluoxetine can cause side effects in some people, but many people have no side effects or only minor ones.

Some of the common side effects will gradually improve as your body gets used to it.

Common side effects

These common side effects of fluoxetine happen in more than 1 in 100 people. There are things you can do to help cope with them:

Feeling sick (nausea)

Try taking fluoxetine with or after food. It may also help to stick to simple meals and avoid rich or spicy food.


Make sure you rest and drink plenty of fluids. Try not to drink too much alcohol. Ask your pharmacist to recommend a painkiller.

Headaches usually go away after the first week of taking fluoxetine. Talk to your doctor if they last longer than a week or are severe.

Being unable to sleep

Take fluoxetine first thing in the morning.


Drink plenty of water or other fluids to avoid dehydration. Signs of dehydration include peeing less than usual or having dark, strong-smelling pee.

Do not take any other medicines to treat diarrhoea without speaking to a pharmacist or doctor.

If you take contraceptive pills and you have severe diarrhoea for more than 24 hours, your contraceptive pills may not protect you from pregnancy. Check the pill packet to find out what to do.

Feeling tired or weak

If fluoxetine makes you feel tired or weak, stop what you’re doing and sit or lie down until you feel better.

Do not drive, ride a bike or use tools or machinery if you’re feeling tired. It’s best not to drink alcohol as it will make you feel worse.

If these symptoms do not go away after a week or two, ask your doctor for advice.

Speak to a doctor or pharmacist if the advice on how to cope does not help and a side effect is still bothering you or does not go away.

Serious side effects

It happens rarely (in less than 1 in 100 people), but some people may have serious side effects when taking fluoxetine.

Book an appointment with your doctor if you:

  • gain weight gain or lose weight without trying
  • get changes in your periods such as heavy bleeding, spotting, or bleeding between periods

Call a doctor or contact 111 straight away if you:

  • get feelings of overwhelming happiness (euphoria), excessive enthusiasm or excitement, or a feeling of restlessness that means you cannot sit or stand still
  • are bleeding from the gums or get bruises that appear without a reason or that get bigger
  • are coughing up blood, or have blood in your pee
  • have black or red poo, or blood in your vomit – these can be signs of bleeding in your stomach

Go to or call 111.

Immediate action required: Call 999 or go to A&E now if:

  • you get chest pain or pressure, or shortness of breath
  • you get severe dizziness or pass out
  • you get painful erections that last longer than 2 hours – this may happen even when you are not having sex
  • you have a fit or seizure
  • you get headaches, have trouble focusing, have memory problems, cannot think clearly, have a seizure or fit, or lose your balance – these can be signs of low sodium levels
  • you have thoughts about harming yourself or ending your life
  • you get any severe bleeding, or bleeding that you cannot stop, such as cuts or nosebleeds that do not stop within 10 minutes

Serious allergic reaction

In rare cases, it's possible to have a serious allergic reaction (anaphylaxis) to fluoxetine.

Immediate action required: Call 999 or go to A&E now if:

  • you get a skin rash that may include itchy, red, swollen, blistered or peeling skin
  • you're wheezing
  • you get tightness in the chest or throat
  • you have trouble breathing or talking
  • your mouth, face, lips, tongue or throat start swelling

You could be having a serious allergic reaction and may need immediate treatment in hospital.

Long-term side effects

Sexual side effects, such as problems getting an erection or a lower sex drive, have been reported after taking fluoxetine for a long time. In some cases, these can continue even after stopping the medicine.

There do not seem to be any lasting harmful effects from taking fluoxetine for many months and years but if you are worried, speak to your doctor.

Other side effects

These are not all the side effects of fluoxetine. For a full list, see the leaflet inside your medicine packet.


You can report any suspected side effect using the Yellow Card safety scheme.

Visit Yellow Card for further information.

Page last reviewed: 10 February 2022
Next review due: 10 February 2025