SSRIs
Symptoms, Causes & Treatment
Everything you need to know about SSRIs: what they are, how they work, which conditions they treat, and how to take them safely.
What Are SSRIs?
SSRIs – Selective Serotonin Reuptake Inhibitors – are the most widely prescribed class of antidepressant medications in the UK and globally. They are used to treat a broad range of mental health conditions and are considered first-line pharmacological treatment for depression, anxiety disorders, OCD, PTSD, and several other conditions in UK clinical guidelines.
SSRIs work by increasing the availability of serotonin – a key neurotransmitter involved in mood regulation, emotional processing, sleep, appetite, and cognition – in the synaptic cleft between nerve cells in the brain. They do this by blocking the reuptake transporter that would normally remove serotonin from the synapse, allowing it to remain active for longer and exert its effects on postsynaptic receptors.
Commonly prescribed SSRIs in the UK include:
- Sertraline (Lustral) – the most commonly prescribed antidepressant in UK primary care; generally well tolerated, low interaction profile, and suitable across a wide range of diagnoses.
- Fluoxetine (Prozac) – one of the longest-acting SSRIs; the preferred choice in young people (under 18) due to its safety profile, and preferred when adherence is a concern due to its long half-life.
- Escitalopram (Cipralex) – highly selective SSRI with good evidence across depression and anxiety; well tolerated.
- Citalopram (Cipramil) – widely used; note that doses are limited to 20mg in those over 65 or with cardiac conditions due to QT prolongation risk.
- Paroxetine (Seroxat) – effective but associated with more significant discontinuation effects; less commonly initiated now.
- Fluvoxamine – used particularly in OCD.
SSRIs are prescription-only medications in the UK. A licensed clinician must assess your symptoms and medical history before prescribing.
What Are the Side Effects of SSRIs?
All SSRIs can cause side effects, though the majority are mild and temporary. Understanding what to expect — and what requires urgent attention — helps people stay on treatment long enough to experience its full benefit.
- Nausea, upset stomach, or diarrhoea — usually mild and settles within 1–2 weeks
- Headache in the first few days of treatment
- Dry mouth
- Increased sweating
- Insomnia or vivid dreams, particularly when starting treatment
- Temporary increase in anxiety or agitation in the first 1–2 weeks
- Reduced sex drive or difficulty with orgasm (may persist)
- Mild tremor or restlessness
- Serotonin syndrome: agitation, confusion, rapid heart rate, high temperature, muscle twitching — rare but serious, especially when combined with other serotonergic drugs
- Hyponatraemia (low sodium): particularly in older adults; presents as confusion, nausea, and seizures
- Increased bleeding risk when combined with NSAIDs or anticoagulants
- QTc prolongation (citalopram, escitalopram): relevant in cardiac patients
- Discontinuation syndrome on abrupt stopping: dizziness, electric-shock sensations, flu-like symptoms
- Increased suicidal ideation in young people under 25 in the first weeks of treatment
Contact a clinician or go to A&E urgently if you develop sudden agitation, confusion, muscle twitching, fever, or rapid heart rate after starting or changing an SSRI — these may indicate serotonin syndrome. If you are a young person under 25 and notice increasing thoughts of self-harm or suicide after starting an SSRI, contact your clinician or a crisis line immediately. Do not stop the medication without speaking to your clinician first.
What Are SSRIs Used For?
SSRIs are prescribed for a wide range of conditions involving serotonin dysregulation. Understanding the specific indication for your prescription, and why a particular SSRI has been chosen, helps support engagement with treatment and realistic expectations about onset and duration.
SSRIs work by blocking the reuptake of serotonin in the synaptic cleft, increasing the availability of serotonin to bind to postsynaptic receptors. This enhances serotonergic neurotransmission in circuits involved in mood regulation, anxiety, and emotional processing.
Depression is the most common indication for SSRIs. They are recommended as first-line pharmacological treatment for moderate to severe depression by NICE, and are also used for mild depression when psychological therapy is unavailable or declined.
SSRIs are first-line medication for Generalised Anxiety Disorder (GAD), Panic Disorder, Social Anxiety Disorder, and OCD. They are also recommended for PTSD, though psychological therapy remains first-line. For OCD, higher doses than those used in depression are typically required.
SSRIs — particularly fluoxetine — are used in Bulimia Nervosa. Some SSRIs have evidence for premenstrual dysphoric disorder (PMDD), premature ejaculation, and certain chronic pain conditions. Off-label use should always be discussed and monitored by a clinician.
All SSRIs share the same core mechanism but differ in half-life, drug interactions, side effect profile, and licensed indications. Fluoxetine has the longest half-life (making discontinuation easier), sertraline has the broadest evidence base, and citalopram/escitalopram have the most favourable interaction profiles but carry a QTc risk at higher doses.
SSRIs interact with a range of commonly used medications. The most clinically significant interactions include: NSAIDs and anticoagulants (increased bleeding), MAOIs (risk of serotonin syndrome — contraindicated), tramadol and triptans (serotonin syndrome risk), and tamoxifen (fluoxetine and paroxetine significantly reduce efficacy).
Key Risk Factors
Which SSRI Is Right for Me?
There is no single ‘best’ SSRI — the choice depends on the condition being treated, your medical history, other medications, and previous treatment response. The table below summarises the key characteristics of each SSRI to help you understand why a particular one may have been prescribed for you.
SSRIs — Dosing & Indications
The table below summarises the key SSRIs available in the UK, their usual dose ranges, and their primary indications. Dosing, duration, and choice of SSRI should always be guided by a clinician based on your individual circumstances.
Supportive Measures
To maximise the benefit of SSRIs, take them at the same time every day — ideally with food to reduce nausea. Set a reminder if needed. Keep regular appointments with your clinician during the first weeks of treatment, particularly if you are under 25. Do not stop the medication because you feel better — early discontinuation significantly increases relapse risk. If you experience side effects you find difficult to tolerate, speak to your clinician before stopping — dose adjustment, switching to a different SSRI, or adding a short-term supportive medication can often resolve the problem.
If SSRIs Are Not Working
For people who do not respond to the first SSRI prescribed, a number of evidence-based options exist. Switching to a different SSRI is often effective, as response to one does not predict response to another. If two adequate SSRI trials have failed, switching to a different antidepressant class (such as an SNRI, mirtazapine, or agomelatine) or augmentation strategies (adding lithium, an antipsychotic, or psychological therapy) may be recommended. Treatment-resistant depression may ultimately require specialist psychiatric input. A clinician can guide you through this process systematically.
When Should You Speak to a Clinician About SSRIs?
You experience sudden agitation, confusion, muscle twitching, rapid heart rate, or high temperature after starting or changing an SSRI — these may indicate serotonin syndrome, which is a medical emergency · A young person under 25 expresses thoughts of suicide or self-harm after starting SSRI treatment · Symptoms of overdose following accidental or intentional ingestion of excess doses.
You are starting an SSRI for the first time and want guidance on what to expect · You are experiencing side effects that are affecting your ability to continue treatment · Your current SSRI does not seem to be working after six weeks at an adequate dose · You want to stop taking an SSRI and need a supervised tapering plan · You are pregnant, planning pregnancy, or breastfeeding and taking or considering an SSRI · You are under 25 and have recently started an SSRI.
How to Take SSRIs Safely
Getting the most from SSRI treatment — and minimising risks — depends on understanding a number of important safety principles. These are not reasons to avoid SSRIs; they are the information you need to take them safely and effectively.
Most early side effects of SSRIs — including nausea, headache, and increased anxiety — are temporary and typically resolve within one to two weeks. Taking the tablet with food, starting at a low dose, and not stopping abruptly all significantly reduce initial side effect burden.
Never stop an SSRI abruptly. Discontinuation syndrome — characterised by dizziness, electric shock sensations (‘brain zaps’), flu-like symptoms, and irritability — can be severe, particularly with paroxetine. Always taper slowly under clinical guidance, reducing the dose gradually over weeks or months.
SSRIs increase bleeding risk, particularly when combined with NSAIDs (ibuprofen, aspirin) or anticoagulants. If you regularly take these medications, discuss the risk with your clinician — a proton pump inhibitor (PPI) may be recommended to protect the stomach lining.
SSRIs must never be combined with MAOIs (such as phenelzine or tranylcypromine) or started within two weeks of stopping them (five weeks for fluoxetine) due to the risk of life-threatening serotonin syndrome. Always inform every clinician prescribing you medication that you are taking an SSRI.
In young people under 25, SSRIs carry a small but real increased risk of suicidal thoughts in the first few weeks of treatment. This does not mean the medication causes suicide — it reflects the activating effects of early treatment in an already vulnerable period. Weekly contact with a clinician or trusted person during the first month is strongly recommended for this age group.
Alcohol amplifies the sedating effects of some SSRIs, worsens depression and anxiety, and can increase the risk of certain side effects. While moderate alcohol is not strictly contraindicated with most SSRIs, it is advisable to limit intake — particularly in the early weeks of treatment.
Speak to a Clinician About SSRIs
If you think you may benefit from SSRI treatment, or if you are currently taking an SSRI and have questions or concerns, The GP Service can help. Through an online consultation with a licensed clinician — from home, at a time that suits you — you can discuss your symptoms, receive a prescription if clinically appropriate, or get support with managing your current medication.
Expert clinical advice, when you need it.
SSRI FAQs
SSRIs (Selective Serotonin Reuptake Inhibitors) are a class of antidepressant medications that work by blocking the reabsorption (reuptake) of serotonin in the brain, increasing its availability in the synaptic cleft. This enhances serotonergic neurotransmission in circuits involved in mood regulation, emotional processing, and anxiety. They are called ‘selective’ because they act primarily on serotonin rather than other neurotransmitters, which gives them a more favourable side effect profile than older antidepressants.
SSRIs are used to treat a wide range of conditions including depression, Generalised Anxiety Disorder (GAD), panic disorder, OCD, PTSD, social anxiety disorder, postnatal depression, bulimia nervosa, and premenstrual dysphoric disorder (PMDD). The specific SSRI prescribed, the dose, and the duration of treatment will depend on the condition being treated. A clinician will select the most appropriate SSRI based on your individual circumstances, medical history, and any other medications you take.
SSRIs typically take four to eight weeks to produce their full therapeutic effect on mood, though some people notice earlier improvements in sleep, energy, or anxiety within the first two weeks. It is important to continue taking the medication as prescribed during this period — stopping early because improvement seems slow is one of the most common reasons for treatment failure. If there is no response after six weeks at an adequate dose, a clinician may suggest adjusting the dose or switching to a different SSRI or antidepressant class.
SSRIs are not addictive in the clinical sense — they do not cause tolerance (needing ever-higher doses to achieve the same effect) or craving. However, they can cause discontinuation syndrome if stopped too quickly, which involves symptoms such as dizziness, nausea, electric shock sensations, and flu-like feelings. This is not addiction — it is a physiological response to the withdrawal of the medication. The solution is always to taper the dose gradually under clinical supervision rather than stopping abruptly.
Sexual side effects — including reduced libido, delayed orgasm, and in men, delayed ejaculation or erectile difficulties — are among the most common persistent side effects of SSRIs. They affect approximately 30–40% of people taking SSRIs. Unlike early side effects such as nausea, sexual side effects often do not resolve with time. Options include dose reduction, switching to a different SSRI or antidepressant (mirtazapine and bupropion have lower rates), adding another medication, or timing doses to minimise impact. Discuss this openly with your clinician — it is a very common concern and there are strategies to help.
SSRIs are generally considered safe in pregnancy — particularly sertraline, which has the largest body of safety data. The decision to continue or start an SSRI in pregnancy involves weighing the risks of untreated depression or anxiety (which carry their own significant risks to mother and baby) against the small risks associated with the medication. This decision should always be made collaboratively with a clinician who can discuss your individual circumstances. Never stop an SSRI abruptly in pregnancy without clinical guidance.
NICE recommends continuing antidepressants for at least six months after achieving full remission from a first depressive episode. For people with recurrent depression (two or more previous episodes), longer-term treatment of two years or more may be recommended. For anxiety disorders and OCD, treatment is typically continued for at least twelve months after symptom remission. The decision to stop should always be made with a clinician and the dose tapered gradually — the speed of tapering depends on the specific SSRI, the dose, and how long you have been taking it.
Serotonin syndrome is a rare but potentially serious reaction caused by excessive serotonergic activity, most commonly resulting from combining two or more serotonergic drugs. Symptoms range from mild (tremor, diarrhoea, agitation) to severe (hyperthermia, muscle rigidity, seizures, cardiac instability). The most dangerous combinations are SSRIs with MAOIs, but risk also exists with tramadol, triptans, lithium, St John’s Wort, linezolid, and some recreational drugs (particularly MDMA). Always inform your clinician and pharmacist of all medications, supplements, and recreational drug use when starting an SSRI.