Fluoxetine

How it works

What does fluoxetine do?

Fluoxetine is a type of antidepressant called a selective serotonin reuptake inhibitor (SSRI).

Research suggests that depression or low mood is more likely to occur when the brain doesn’t have enough serotonin.

Serotonin (also called '5HT') is a naturally-occurring chemical messenger (or “neurotransmitter”) that has an important role in areas of the brain that control mood and thinking.

Selective serotonin reuptake inhibitors (SSRIs) - like fluoxetine - are thought to work by increasing levels of serotonin in the brain. They do this by blocking the recycling of released serotonin back into the nerve endings.

Fluoxetine is often prescribed alongside a talking therapy.

How long will fluoxetine take to start working?

Antidepressants like fluoxetine can start to work on depression within the first two weeks of treatment, with the improvements continuing over the following weeks,

For anxiety, antidepressants like fluoxetine can take slightly longer to work. For some people, anxiety briefly increases at the start of treatment, but the anxiety does decrease with continued treatment.

How long will I need to take fluoxetine?

Most people take fluoxetine for at least six to 12 months after they start to feel better.

If you have suffered from depression in the past, you should keep taking this medication for at least two years after you start to feel better.

You and your doctor should talk about how long you need to take fluoxetine before you begin your treatment.

If you stop taking the fluoxetine too soon, the chance that your mental health symptoms will come back increases.

Taking fluoxetine

You should take fluoxetine only as agreed with your doctor.

You will get the best effect from fluoxetine if you take it every day at the dose prescribed by your doctor.

Your doctor might start you on a low dose and then increase it to your full dose slowly over two to four weeks.

Make sure that you know your dose. If it's not written on the label, check with your pharmacist or doctor.

Young people aged eight to 18 may start taking fluoxetine at a lower dose using the liquid, and then move on to the capsules if the dose increases.

It is usually best to take fluoxetine in the morning. This is to reduce the chance of it causing insomnia (difficulty sleeping). However, it is important to choose a time each day that you can always remember. This could be when you wake up, a mealtime, or when you brush your teeth.

Fluoxetine is best taken after food.

Swallow the capsule with a drink of water - if you chew it, it tastes bitter.

What if I miss a dose?

If you remember later during the day, take it as soon as possible.

You may find it difficult to sleep if you take it towards bedtime.

If you forget to take it by the time of your next dose, just take the next dose.

Do not take a double dose.

What will happen if I forget to take my fluoxetine?

If you forget to take it for a few days, you may start getting withdrawal symptoms and should talk to your doctor about it.

Stopping the use of fluoxetine

Stopping fluoxetine suddenly can sometimes cause side effects, which are usually mild, but for a few people can be severe. Also, if you stop taking fluoxetine too soon, the chance that your mental health symptoms will come back increases.

Once you start taking an SSRI, the brain adjusts to having a new level of serotonin around. If you stop taking the SSRI suddenly, the balance starts to change again. You could get some symptoms from the change.

Fluoxetine is less likely to cause withdrawal effects upon stopping than other antidepressant medicines. This is because your body takes longer to get rid of it.

You can stop taking fluoxetine safely with your doctor’s help. Many people who take 20mg or less of fluoxetine can stop taking it without problems. For people on higher doses of fluoxetine, the dose may be stepped down over a few weeks to reduce the chance of withdrawal effects.

Fluoxetine and other antidepressants are not addictive. Although there can be withdrawal effects when they are stopped, you will not have cravings for or get ‘hooked’ on an antidepressant.

Some of the withdrawal effects you might get when fluoxetine is stopped include:

• flu-like symptoms such as chills, muscle aches, excessive sweating, headache, and feeling or being sick
• feeling unusually tired or weak
• sleep disturbances such as difficulty sleeping or vivid dreams
• electric shock-like sensations - especially down the spine (back)
• dizziness, especially when moving
• feeling anxious, restless, irritable, or agitated
• difficulty remembering things or concentrating on things

These symptoms should stop after two weeks for most people, but some people can get them for a few months.

The information on this page was reviewed by the College of Mental Health Pharmacy in March 2020.

Safety headlines

If you have taken more fluoxetine than the dosage recommended by the doctor who prescribed it to you, you must get medical help immediately – even if you do not feel any different.

While taking fluoxetine, some people may think about hurting themselves or taking their own lives. You must go straight to hospital with your medicine if you have any of these thoughts.

Fluoxetine can also cause rare but serious side effects: allergic reactions (difficulty breathing, swelling of your face or throat, itching skin lumps), rashes or blotches on your skin, and a rare condition called serotonin syndrome (see 'Side Effects'). Go to a hospital with your medicine if you get any of these symptoms.

Do not take fluoxetine if you have taken a monoamine oxidase inhibitor antidepressant (MAOI) like moclobemide, phenelzine, isocarboxazid or tranylcypromine in the last 14 days.

Stopping fluoxetine suddenly can sometimes cause withdrawal effects, which are usually mild, but for a few people can be severe. Also, if you stop taking fluoxetine too soon, the chance that your mental health symptoms will come back increases. See your doctor if you want to stop fluoxetine or have withdrawal effects after stopping it.

You might not be able to concentrate very well, and may feel sleepy, in the first few days after taking fluoxetine. Do not drive a car, ride a bike or operate machines until you see how this affects you.

If you are pregnant, or thinking of becoming pregnant, please read the pregnancy section (see ‘Side Effects') because fluoxetine may affect the developing baby.

When to go to the hospital

If you have taken more fluoxetine than the dosage recommended by the doctor who prescribed it to you, you must get medical help immediately – even if you do not feel any different.

Go to A&E. Take your medicine with you to show to the doctors. Tell them how much you have taken. Get a friend or family member to go with you, if you can, just in case you feel ill on the way.

You might get any of the following signs:

• feeling sick or being sick
• having seizures (fits)
• an irregular heartbeat or a heart attack
• breathing difficulties
• a change in the way you feel, ranging from agitation to falling into a coma

You are also at risk of getting ‘serotonin syndrome’ (symptoms include a high fever, agitation, confusion, trembling, shivering and excessive sweating or weird movements of your muscles). This is rare, but you should watch out for it.

If you have any thoughts of taking your own life or of other ways of hurting yourself, go straight to a hospital with your medicine. This may be a side effect and is more common at the start of treatment. Thoughts of suicide are more likely to occur if you are under 25 years old. Fluoxetine is safer than other antidepressants for people under the age of 18.

You must stop taking Fluoxetine and go straight to hospital if you have any of the following symptoms:

• tiredness, confusion, headache, irritability, feeling or being sick (nausea and vomiting), and muscle twitching. These can be symptoms of a low blood level of sodium, but some of these are also symptoms of serotonin syndrome
• rashes; blotches; itching; blistering; redness; peeling; or ulcers on your skin, in your mouth, or in your genital area. These can be symptoms of a rare but serious skin reaction
• difficulty breathing, swelling of your face or throat, and itching skin lumps. These may be symptoms of an allergic reaction
• having seizures (fits)
• feeling very excited or ‘high’

Go to a doctor or hospital straight away but do not stop taking your fluoxetine if you get the following symptoms:

• your behaviour changes because you feel irritated or agitated
• you get an erection that is painful and lasts for a long time (priapism)

Side effects

Please do not be worried by the side effects listed on this page. Many people take fluoxetine without any side effects or only a few mild side effects. Starting with a lower dose can sometimes help if side effects do occur.

Side effects that do appear should disappear or get better after a few days. If they do not, you should go back to your doctor.

Do not stop taking the fluoxetine until you talk to your doctor, or you may get withdrawal symptoms as well.

Very common side effects when taking fluoxetine (affecting more than one in ten people) include:

• insomnia (sleep problems)
• headache and feeling tired
• diarrhoea (loose poo)
• nausea (feeling sick)

Common side effects of taking fluoxetine (affecting up to one in ten people) include:

• not feeling hungry
• weight loss
• nervousness, anxiety, restlessness, poor concentration, feeling tense 
• decreased sex drive or sexual problems, including difficulty maintaining an erection (staying hard)
• sleep problems, unusual dreams, tiredness or sleepiness
• dizziness
• change in taste, or dry mouth
• uncontrollable shaking movements
• blurred vision
• heartbeat feels quick and uneven
• flushing, sweating more, feeling shaky or chills
• yawning
• indigestion, being sick
• rash, itching lumps (hives, urticaria), other skin itching
• joint pain
• needing to wee more often
• unexplained vaginal bleeding

There are other side effects that you can get when taking this medicine – we have only included the most common ones here.

Please look at the leaflet inside your medicine box, or ask a doctor or pharmacist, if you want to know whether you are getting a side effect from your medicine.

If you do get a side effect, please think about reporting it via the Yellow Card Scheme.

Body

Your weight can be affected by fluoxetine.

A side effect of fluoxetine can be not feeling as hungry as normal, which might lead to weight loss.

It is very difficult to know how this will affect each person who takes it.

Talk to your doctor about this if it worries you.

Sleep

You can feel drowsy in the first few days of taking fluoxetine. However, it should get better after the first week or two. You may become more anxious, or it may make you irritable. This should settle after a couple of weeks.

If you feel very sleepy, and you’ve been taking it for more than a month, you should go back to the doctor and see what else you could do.

Sex and fertility

Sex

Fluoxetine can have side effects that might affect your sex life. These include:

• painful erections that last for a long time (this is rare but serious - visit a hospital or see your doctor straight away if you experience this side effect), or problems getting an erection (getting hard) and ejaculating (coming)
• bleeding from the vagina
• difficulty reaching orgasm the same way as before
• some growth of the breasts and some milk flow, regardless of gender
• you may have a lower sex drive

Fluoxetine has been known to slow growth and delay sexual development (puberty) in a small number of children and young people. Your doctor should check your growth while you are taking it. If this worries you, talk to your doctor or pharmacist about it.

These effects should pass after the first couple of weeks. If they do not, and this is a problem for you, go back to the doctor and see what else you could try.

The good effects of fluoxetine may, after a while, have a positive impact on your sex life as your mood lifts and you become interested in life and relationships again.

Fertility

Fluoxetine does not seem to affect human fertility.

Talk to your doctor about your fluoxetine if you are trying to get pregnant.

Pregnancy, post-natal and breastfeeding

Pregnancy

If you become pregnant while you are on fluoxetine, you should carry on taking the medicine and go back to your doctor as soon as possible, to see if you should change or stop your medicine.

There is a slightly higher risk of problems in the developing baby if you take fluoxetine it in the early stages of pregnancy. However, fluoxetine and related antidepressants (known as SSRIs) are thought to be better choices if you need antidepressant medicine.

Remember that you need to stay well through your pregnancy, and you may need a medicine to help you to do that.

Fluoxetine may cause heart problems in the developing baby, and other symptoms in new-born babies.

If you and your doctor agree that you will continue taking fluoxetine during your pregnancy, then you should tell your midwife that you are taking it before you give birth.

Post-natal

If fluoxetine is taken in the last five months of a pregnancy, it can cause a serious condition called persistent pulmonary hypertension of the new-born (PPHN). This can make the baby breathe faster and look a bit blue in colour. PPHN affects around three in 1,000 babies born to mums who take SSRIs. This compares with a rate of two in 1,000 among babies born to mums who do not take SSRIs.

PPHN appears in the first 24 hours after birth. You will need help from the midwife and doctors, so it is better if they are looking out for symptoms.

The newborn baby may also develop withdrawal effects, which might not appear straight away but might develop over the first few days of life. These could include:

• being irritable and crying a lot
• having difficulty sleeping or sucking
• poor weight gain

Breastfeeding

Fluoxetine is passed to the baby in breast milk, and side effects have been seen in breastfed babies. The main one is likely to be colic.

The amount of fluoxetine in breast milk is usually around 7% of the mum’s dose, but it can build up over time. This is because fluoxetine is hard for your baby to get rid of. This does not mean breastfeeding will be a problem, but do look out for side effects.

If your baby was premature or has health problems, then you may be better off not breastfeeding as your baby will likely struggle even more to get rid of the fluoxetine.

Talk to your doctor or midwife about your feeding options.

Driving and transport

Do not drive or ride a bike just after you start taking fluoxetine.

Taking fluoxetine may affect your ability to do things like drive a car, ride a bike, use machines, or anything else that needs a lot of focus.

It might be best to stop doing these things for the first few days, until you know how it affects you, or until the effects pass.

Do not worry - most people drive as normal while taking fluoxetine.

School and exams

Try not to take fluoxetine for the first time just before your exams.

Taking fluoxetine may affect your ability to do things that need a lot of focus, like exams.

You should talk to your doctor about any future exams if you are starting fluoxetine.

You might decide together to delay starting it until you have done them. If they are more than a month away, however, you might find that it is better to start fluoxetine to lift your mood and improve your motivation to study.

Do not worry - most people do exams as normal while taking fluoxetine.

Friends and family

You may want to let your family and friends know you are taking fluoxetine so they can support you and help you look out for side effects.

For guidance on this, check out our page on getting support with your medication.

Sport

Fluoxetine is not a banned substance in sport.

Taking fluoxetine may affect your ability to do things like riding a bike, competitive gymnastics, or anything else that needs a lot of focus.

It might be best to stop such sports for the first few days, until you know how it affects you or the effects get better.

Do not worry - most people play sports as normal while taking fluoxetine.

Alcohol and street drugs

Alcohol

You can continue to drink some alcohol while taking fluoxetine, but it is best to do so in moderation. Fluoxetine can sometimes cause drowsiness as a side effect, so it is possible that alcohol might make you feel more drowsy than usual.

Drinking alcohol every day, however, can make the symptoms of depression worse and you will not feel the benefit of your medicine.

Side effects might make you sleepy or you might lose your focus when you first start taking fluoxetine.

This could be dangerous if you drive or use machines or do anything that needs a lot of focus.

During the first few days, it might be best to stop drinking alcohol until you see how the medicine affects you, or until the effects pass.

Street drugs

Be careful if you are also using street drugs.

Cannabis can have unpredictable effects when taken with fluoxetine, so great care is needed.

Cannabis and other drugs may have their own side effects on your mental health, like anxiety or psychosis. For more information, have a look at our drugs and alcohol page.

Methadone and fluoxetine together can seriously affect your heart so these should only be combined under doctor supervision.

Fluoxetine has been shown to dampen down the 'high' of cocaine.

Taking fluoxetine with cocaine, ecstasy or amfetamines could bring on serotonin syndrome. You could get a high temperature/fever, agitation, confusion, trembling or weird muscle movements. You need to go to hospital if this happens. Tell the doctor everything that you have taken.

Fluoxetine can produce a false positive test for amfetamines and LSD on a urine drug screen. Talk to your doctor about this if it is a problem for you.

Prescription medicines

Fluoxetine does not mix well with some other medicines and drugs.

Do not take fluoxetine if you take an antidepressant medicine called a monoamine oxidase inhibitor (MAOI), or if you have taken one in the last two weeks. MAOIs include moclobemide, isocarboxazid, phenelzine and tranylcypromine.

Tell your doctor or pharmacist before you take fluoxetine if you are prescribed any other medication, to check that the combination is safe. 

Before you start taking fluoxetine, tell your doctor if you are taking any other medications, including things you have bought over the counter for common illnesses like colds and flu or topical applications that you put on your skin.

References and further reading

For more helpful links and information, have a look at our references and further reading page.