SSRIs (selective serotonin reuptake inhibitors)


SSRIs (selective serotonin reuptake inhibitors)


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

They are 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 certain other mental health conditions, including:

generalised anxiety disorder (GAD)

obsessive compulsive disorder (OCD)

panic disorder

severe phobias, such as agoraphobia and social phobia


post-traumatic stress disorder (PTSD)

SSRIs can also be used to treat people with certain other conditions, such as premature ejaculation, premenstrual syndrome (PMS), fibromyalgia and irritable bowel syndrome (IBS).


How they work

It's thought that SSRIs work by increasing the levels of a chemical called serotonin 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 are prescribed, you will start on the lowest possible dose thought necessary to improve your symptoms.

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

If you take an SSRI for four to six 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 six months, although longer courses are sometimes recommended and some people with recurrent problems may be advised to take them indefinitely.


Things to consider

SSRIs are not suitable for everyone. They are not usually recommended if you are pregnant, breastfeeding or under 18 because there is an increased risk of serious side effects, although 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 diabetes, epilepsy 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 medications you need to avoid.


Side effects

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

Common side effects of SSRIs can include:

feeling agitated, shaky or anxious

feeling or being sick


blurred vision

low sex drive

difficulty achieving orgasm during sex or masturbation

in men, difficulty obtaining or maintaining an erection (erectile dysfunction)

You will 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 troublesome or persistent side effects.


Cautions and interactions 

Selective serotonin reuptake inhibitors (SSRIs) are not suitable for everyone as they can cause problems if you have other health conditions or if they are 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 are in a manic phase (a period of extremely excitable moods), although they can be useful for depressive phases

a bleeding disorder, such as haemophilia

type 1 or type 2 diabetes

epilepsy – SSRIs should only be taken if your epilepsy is well controlled, and the medication should be stopped if your epilepsy gets worse

narrow angle glaucoma (increased pressure in the eye)

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 are not usually recommended during pregnancy, especially during the first three 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 is not clear whether SSRIs do definitely cause these complications. Most experts think that if SSRIs do increase risks of complications in pregnancy, the increase is probably small.

If you are pregnant and think you may be depressed, you should discuss risks and benefits of using SSRIs with your doctor. If SSRIs are recommended, you will 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 are taking SSRIs.



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

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

If you are prescribed SSRIs when breastfeeding, paroxetine or sertraline are normally recommended.

Children and young people

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

Concerns have also been raised that 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) alone have not helped. In these 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 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..

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

non-steroidal anti-inflammatory drugs (NSAIDs)– a common type of painkiller that includes ibuprofen, diclofenac or naproxen

antiplatelets – a type of medication used to prevent blood clots, such as low-dose aspirin and clopidogrel

theophylline – a medication used to treat asthma

clozapine and pimozide – medications used to treat schizophrenia and psychosis

lithium – a medication used to treat severe depression and bipolar disorder

triptans – a type of medication used to treat migraines, such as naratriptan, sumatriptan and zolmitriptan

other antidepressants – including tricyclic antidepressants (TCAs), monoamine oxidase inhibitors (MAOIs) and serotonin-norepinephrine reuptake inhibitors (SNRIs)

However, this is not 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 (PIL) that comes with your SSRI medication to see if there are any medications you should avoid. Online versions of most PILs can be found in the medicines A-Z.

If in doubt, your pharmacist or GP should be able to advise you.


Interactions with food and drink

Alcohol is not usually recommended if you are taking an SSRI as 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.

Therefore, you should 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 is evidence of its effectiveness, 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 SSRI 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 this 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 will usually have to take one to three tablets a day.

It usually takes between two and four weeks before you begin to notice the effects of SSRIs. You will 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 four to six weeks. They may recommend increasing your dose or trying an alternative antidepressant.

It is usually recommended that a course of SSRIs lasts at least six months to prevent your condition recurring when you stop. However, if you have experienced previous episodes of depression, a two-year course may be recommended and some people with recurrent problems are advised to carry on taking medication indefinitely.


Missed or extra doses

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

If you do miss one of your doses, take it as soon as you remember unless it is 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 is not possible, contact your local out of hours service, or call 111.

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


Stopping SSRIs

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

stomach upsets

flu-like symptoms



sensations in the body that feel like electric shocks

seizures (fits)

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


Side effects of SSRIs 

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

You will usually see your doctor every few weeks during when you first start taking SSRIs to discuss how well the medication is working, although 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 is not an exhaustive list and some of these will not all necessarily apply to the specific SSRI you are taking. For information on the side effects for a particular SSRI, check the information leaflet that comes with your medication or search for your medication in the medicines A-Z.


Common side effects

Common side effects of SSRIs can include:

feeling agitated, shaky or anxious

feeling or being sick


diarrhoea or constipation

loss of appetite and weight loss


blurred vision

dry mouth

excessive sweating

not sleeping well (insomnia) or drowsiness


low sex drive

difficulty achieving orgasm during sex or masturbation

in men, difficulty obtaining or maintaining an erection (erectile dysfunction)

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


problems with movement, such as stiffness or shaking

seeing or hearing things that are not real (hallucinations)

being unable to pass urine

If you vomit blood, have blood in your stools or have problems passing urine, speak to your doctor or go to hospital immediately.


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 called serotonin in your brain become too high. It is 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:



muscle twitching




If you experience the symptoms listed above, you should stop taking the medication and seek immediate advice from your GP or specialist. If this is not possible, call 111.

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 hyponatraemia. This may lead to a build-up of fluid inside the cells of the body, which can be potentially dangerous.

This side effect can occur as SSRIs can block the effects of a hormone that helps 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 hyponatraemia can cause symptoms similar to depression or side effects of SSRIs, such as:

feeling sick


muscle pain

reduced appetite


More severe hyponatraemia can cause the following symptoms:

feeling listless and tired



psychosis (being unable to tell the difference between reality and your imagination)

seizures (fits)

The most serious cases of hyponatraemia can cause you to stop breathing or enter a coma.

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

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


Suicidal thoughts

Some people experience 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 are taking SSRIs.

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


Reporting side effects

The Yellow Card Scheme allows you to report suspected side effects from any type of medicine that you are taking. It is run by a medicines safety watchdog called the Medicines and Healthcare products Regulatory Agency (MHRA).

See the website of the Yellow Card Scheme for more information.