Selective serotonin reuptake inhibitors (SSRIs) are a widely used type of antidepressant medication.

They're mainly prescribed to treat depression, particularly persistent or severe cases, and are often used in combination with a talking therapy such as cognitive behavioural therapy (CBT).

SSRIs are usually the first choice medication for depression because they generally have fewer side effects than most other types of antidepressant.

As well as depression, SSRIs can be used to treat a number of other mental health conditions, including:

SSRIs can sometimes be used to treat other conditions, such as premature ejaculation, premenstrual syndrome (PMS), fibromyalgia and irritable bowel syndrome (IBS). Occasionally, they may also be prescribed to treat pain.

How SSRIs work

It's thought that SSRIs work by increasing serotonin levels in the brain.

Serotonin is a neurotransmitter (a messenger chemical that carries signals between nerve cells in the brain). It's thought to have a good influence on mood, emotion and sleep.

After carrying a message, serotonin is usually reabsorbed by the nerve cells (known as "reuptake"). SSRIs work by blocking ("inhibiting") reuptake, meaning more serotonin is available to pass further messages between nearby nerve cells.

It would be too simplistic to say that depression and related mental health conditions are caused by low serotonin levels, but a rise in serotonin levels can improve symptoms and make people more responsive to other types of treatment, such as CBT.

Doses and duration of treatment

SSRIs are usually taken in tablet form. When they're prescribed, you'll start on the lowest possible dose thought necessary to improve your symptoms.

SSRIs usually need to be taken for 2 to 4 weeks before the benefit is felt. You may experience mild side effects early on, but it's important that you don't stop taking the medication. These effects will usually wear off quickly.

If you take an SSRI for 4 to 6 weeks without feeling any benefit, speak to your GP or mental health specialist. They may recommend increasing your dose or trying an alternative antidepressant.

A course of treatment usually lasts for at least 6 months, although longer courses are sometimes recommended and some people with recurrent problems may be advised to take them indefinitely.

Read more about SSRI doses.

Things to consider

SSRIs aren't suitable for everyone. They're not usually recommended if you're pregnant, breastfeeding or under 18, because there's an increased risk of serious side effects. However, exceptions can be made if the benefits of treatment are thought to outweigh the risks.

SSRIs also need to be used with caution if you have certain underlying health problems, including diabetesepilepsy and kidney disease.

Some SSRIs can react unpredictably with other medicines, including some over-the-counter painkillers and herbal remedies, such as St John’s wort. Always read the information leaflet that comes with your SSRI medication to check if there are any medicines you need to avoid.

Read more about the cautions and interactions of SSRIs.

Side effects

Most people will only experience a few mild side effects when taking SSRIs. These can be troublesome at first, but they'll generally improve with time.

Common side effects of SSRIs can include:

  • feeling agitated, shaky or anxious
  • feeling or being sick
  • dizziness
  • blurred vision
  • low sex drive
  • difficulty achieving orgasm during sex or masturbation
  • in men, difficulty obtaining or maintaining an erection (erectile dysfunction)

You'll usually need to see your doctor every few weeks when you first start taking SSRIs to discuss how well the medication is working. You can also contact your doctor at any point if you experience any troublesome or persistent side effects.

Read more about the side effects of SSRIs.

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Cautions and interactions

Selective serotonin reuptake inhibitors (SSRIs) aren't suitable for everyone. They can cause problems if you have other health conditions or if they're taken alongside other medications.

Some of the issues you and your doctor will need to bear in mind when taking SSRIs, or when considering using them, are described below.

Medical conditions

SSRIs may not be suitable if you have any of the following conditions:

  • bipolar disorder and you're in a manic phase (a period of extremely excitable mood), although they can be useful for depressive phases
  • a bleeding disorder, such as haemophilia
  • type 1 diabetes or type 2 diabetes
  • epilepsy – SSRIs should only be taken if your epilepsy is well controlled, and they should be stopped if your epilepsy gets worse
  • narrow angle glaucoma
  • serious kidney, liver or heart problems

SSRIs may need to be used with caution or not at all if you have one of these conditions, because the medication could increase your chance of experiencing serious side effects.


As a precaution, SSRIs aren't usually recommended during pregnancy, particularly during the first 3 months (the first trimester). This is because there may be a risk to the baby.

However, exceptions can be made if the risk posed by depression (or another mental health condition) outweighs the potential risks of treatment.

Possible risks of taking SSRIs during pregnancy include:

  • loss of the pregnancy
  • birth defects affecting the baby’s heart (congenital heart disease)
  • the baby being born with a rare condition called persistent pulmonary hypertension in the newborn (PPHN), which causes breathing and circulation problems

However, it isn't clear whether SSRIs definitely cause these complications. Most experts think that if SSRIs do increase the risks of complications during pregnancy, the increase is probably small.

If you're pregnant and think you may be depressed, you should discuss the risks and benefits of using SSRIs with your doctor. If SSRIs are recommended, you'll usually be prescribed fluoxetine, citalopram or sertraline because these are thought to be relatively safe to use.

You should also speak to your doctor for advice if you become pregnant while you're taking SSRIs.


As a precaution, SSRIs aren't usually recommended if you're breastfeeding. This is because the medication may be passed to your baby in your breast milk.

However, SSRIs may be used if it's thought that the benefits of treatment and benefits of breastfeeding your baby outweigh the potential risks.

If you're prescribed SSRIs when breastfeeding, paroxetine or sertraline are usually recommended.

Children and young people

SSRIs aren't usually recommended for children and young people under the age of 18. This is because there's evidence of an increased risk of self-harm and thoughts about suicide in this age group.

There are also concerns that the use of SSRIs could affect brain development in children and young people.

However, children and young people under the age of 18 may be offered an SSRI if talking therapies, such as cognitive behavioural therapy (CBT), alone haven't helped. In such cases, an SSRI will only be prescribed in addition to a talking therapy and treatment must be supervised by a psychiatrist (a doctor who specialises in treating mental health conditions).

If an SSRI is recommended, fluoxetine is usually the first choice.

Driving and operating machinery

Some SSRIs can cause dizziness, drowsiness and blurred vision, particularly when you first start taking them.

If you experience these problems, you should avoid driving or using heavy tools and machinery.

Interactions with other medications

SSRIs can react unpredictably with certain other medications (known as "interacting"), potentially increasing the risk of side effects such as bleeding or a problem known as "serotonin syndrome".

Read more about the side effects of SSRIs.

Some of the medications that can interact with some SSRIs include:

However, this isn't an exhaustive list of all the medications that can interact with SSRIs, and not all of these interactions apply to all types of SSRI.

You should always make sure you carefully read the patient information leaflet that comes with your SSRI medication to see if there are any medications you should avoid. If in doubt, your pharmacist or GP should be able to advise you.

Interactions with food and drink

Alcohol isn't usually recommended if you're taking an SSRI, because it can increase any drowsiness you may experience and can make feelings of depression worse. The SSRI, fluvoxamine, is also known to enhance the effects of caffeine, so people who drink large amounts of caffeine may experience unpleasant symptoms such as palpitations, feeling sick, restlessness and insomnia.

You should therefore avoid drinking large amounts of caffeinated drinks, such as tea, coffee, energy drinks and cola, while taking fluvoxamine.

St John’s wort

St John’s wort is a popular herbal remedy promoted for the treatment of depression.

While there's some evidence that St John's wort may help mild to moderate depression, many experts advise against using it because the amount of active ingredient can vary significantly and you can never be sure what sort of effect it will have on you.

Taking St John's wort alongside SSRIs can also potentially cause serious health problems.

About your dose

When prescribing selective serotonin reuptake inhibitors (SSRIs), your doctor will usually select the lowest possible dose thought necessary to improve your symptoms.

This approach is intended to reduce the risk of side effects. If the prescribed dose proves ineffective, it can be gradually increased.

SSRIs are usually taken in tablet form. Depending on the type of SSRI prescribed and the severity of your depression, you'll usually have to take 1 to 3 tablets a day.

It will usually take 2 to 4 weeks before you begin to notice the effects of SSRIs. You'll have regular meetings with your doctor when you first start taking SSRIs and you should let them know if you haven't noticed any improvement after 4 to 6 weeks. They may recommend increasing your dose or trying an alternative antidepressant.

It's usually recommended that a course of SSRIs lasts at least 6 months to prevent your condition recurring when you stop. However, if you've experienced previous episodes of depression, a 2-year course may be recommended. Some people with recurring problems are advised to carry on taking medication indefinitely.

Missed or extra doses

It's important not to miss any of your doses, because this could make your treatment less effective.

If you do miss a dose, take it as soon as you remember, unless it's almost time to take your next dose, in which case you should just skip the missed dose. Don't take a double dose to "make up" for the one you missed.

If you take more tablets than prescribed, contact your GP as soon as possible for advice. If this isn't possible, contact your local out of hours service, or call NHS 24 111 service.

Taking a double dose is unlikely to be harmful, but you should only do so if advised by a medical professional.

Stopping SSRIs

You shouldn't suddenly stop taking SSRIs, even if you feel better. Stopping suddenly can lead to withdrawal symptoms such as:

If your GP or mental health specialist decides to stop your course of SSRIs, they'll reduce the dose gradually over a few weeks.

Side effects

The side effects of selective serotonin reuptake inhibitors (SSRIs) can be troublesome at first, but most improve with time.

In general, SSRIs are better tolerated than most other types of antidepressants. The majority of people will only experience a few mild side effects when taking them.

It's important to persist with treatment, even if you're affected by side effects, as it will take several weeks before you begin to benefit from treatment. With time, you should find that the benefits of treatment outweigh problems related to side effects.

You'll usually see your doctor every few weeks when you first start taking SSRIs to discuss how well the medication is working. However, you can contact your doctor at any point if you experience any particularly troublesome or persistent side effects.

Some of the main side effects of SSRIs are described below, but this isn't an exhaustive list and some of these won't necessarily apply to the specific SSRI you're taking. For information about the side effects of a particular SSRI, check the information leaflet that comes with your medication.

Common side effects

Common side effects of SSRIs can include:

These side effects should improve over time, although some – such as sexual problems – can persist.

Less common side effects

Less common side effects of SSRIs can include:

  • bruising or bleeding easily, including vomiting blood or blood in your stools
  • confusion
  • movement problems, such as stiffness or shaking
  • seeing or hearing things that aren't real (hallucinations)
  • being unable to pass urine

Speak to your doctor or go to your nearest hospital immediately if you vomit blood, have blood in your stools, or have problems passing urine.

Serotonin syndrome

Serotonin syndrome is an uncommon, but potentially serious, set of side effects linked to SSRIs.

Serotonin syndrome occurs when the levels of a chemical in your brain called serotonin become too high. It's usually triggered when you take an SSRI in combination with another medication (or substance) that also raises serotonin levels, such as another antidepressant or St John’s wort.

Symptoms of serotonin syndrome can include:

  • confusion
  • agitation
  • muscle twitching
  • sweating
  • shivering
  • diarrhoea

If you experience the symptoms listed above, you should stop taking the medication and seek immediate advice from your GP or specialist. If this isn't possible, call NHS 24 111 service.

Symptoms of severe serotonin syndrome include:

  • a very high temperature (fever)
  • seizures (fits)
  • irregular heartbeat (arrhythmia)
  • loss of consciousness

If you or someone you know experience symptoms of severe serotonin syndrome, seek emergency medical help immediately by dialling 999 and asking for an ambulance.


Elderly people who take SSRIs may experience a severe fall in sodium (salt) levels known as hyponatremia. This may lead to a build-up of fluid inside the body's cells, which can be potentially dangerous.

This side effect occurs because SSRIs can block the effects of a hormone that helps to regulate levels of sodium and fluid in the body. Elderly people are vulnerable because fluid levels become more difficult for the body to regulate.

Mild hyponatremia can cause symptoms similar to depression or side effects of SSRIs, such as:

  • feeling sick
  • headache
  • muscle pain
  • reduced appetite
  • confusion

More severe hyponatremia can cause the following symptoms:

  • feeling listless and tired
  • disorientation
  • agitation
  • psychosis
  • seizures (fits)

The most serious cases of hyponatremia can cause you to stop breathing or go into a coma.

If you suspect that you or someone in your care has mild hyponatremia, call your GP for advice and stop taking SSRIs for the time being. If you suspect severe hyponatremia, call 999 to request an ambulance.

Hyponatremia can be treated by feeding a sodium solution into the body through an intravenous drip.

Suicidal thoughts

Some people have suicidal thoughts and a desire to self-harm when they first take SSRIs. Young people under 25 seem particularly at risk.

Contact your GP or go to hospital immediately if you have thoughts of killing or harming yourself at any time while you're taking SSRIs.

It may be useful to tell a relative or close friend that you've started taking antidepressants and ask them to read the leaflet that comes with your medication. Ask them to tell you if they think your symptoms are getting worse, or if they're worried about changes in your behaviour.

Last updated:
23 February 2023