PTCB Guide to SSRIs

Also called Selective serotonin reuptake inhibitors · suffix no single suffix · updated July 15, 2026

What gets tested

• Matching SSRI brand names to generics (Zoloft is sertraline, Prozac is fluoxetine, Lexapro is escitalopram)
• The serotonin reuptake (SERT) mechanism and the conditions SSRIs treat
• That SSRIs take several weeks to reach full effect
• The serotonin syndrome interaction with MAOIs, triptans, tramadol, linezolid, and St John's wort
• The boxed warning for suicidal thoughts and behaviors: the risk was highest in patients 24 and younger, and all patients starting an antidepressant are monitored closely
• Telling SSRIs apart from SNRIs such as duloxetine and venlafaxine
• That fluvoxamine is still paired with Luvox on drug lists even though the Luvox and Luvox CR brands are no longer marketed in the United States, so it is dispensed as a generic
• The citalopram maximum daily dose (40 mg, or 20 mg for patients over 60) that exists because of QT prolongation

What is not tested

You do not need to memorize receptor binding affinities, week by week titration schedules, or the full substrate list for every CYP450 enzyme. Do not skip strengths and doses altogether, though: PTCE knowledge area 1.4 covers strengths and doses, dosage forms, routes of administration, special handling and administration instructions, and duration of therapy, so limits like the citalopram maximum of 40 mg per day (20 mg per day for patients over 60) and the once daily versus twice daily difference between fluvoxamine forms are fair game. Knowing that fluoxetine and paroxetine are the strong CYP2D6 inhibitors is also worth the time, even though the wider enzyme tables are not. What the exam does not ask you to do is make prescribing decisions: it focuses on recognizing the drugs, the class and its uses, the signature side effects, and the high-yield serotonin syndrome interaction.

Why ssris matter

SSRIs are among the most commonly dispensed medications in the United States, with sertraline and escitalopram sitting near the top of dispensing lists. They appear constantly in the Medications domain of the PTCE, and serotonin syndrome is a classic exam and real-world safety point, so learning this class well is a high-yield use of study time.

SSRIs at a glance

Mechanism of action

SSRIs block the serotonin transporter (SERT) on the presynaptic nerve terminal, so less serotonin (5-HT) is pulled back into the neuron and more stays in the synaptic cleft to stimulate postsynaptic receptors. Unlike older antidepressants, SSRIs have little effect on norepinephrine, dopamine, histamine, or acetylcholine (paroxetine is the exception: it has meaningful anticholinergic activity), which is why they cause fewer side effects than tricyclic antidepressants (TCAs) and monoamine oxidase inhibitors (MAOIs). The mood benefit is not immediate: it can take several weeks (up to about 6 weeks) of steady daily dosing before a patient feels the full effect. The SSRIs used in the United States are fluoxetine, sertraline, paroxetine, fluvoxamine, citalopram, and escitalopram. Vilazodone (Viibryd) is sometimes grouped with them, but its FDA label describes it as both a selective serotonin reuptake inhibitor and a 5-HT1A receptor partial agonist, so it is usually classed separately, and unlike the six above it has to be taken with food.

Common and important side effects

• Nausea and other gastrointestinal upset, most common early in treatment
• Sexual dysfunction (lowered desire, delayed orgasm, erectile problems). The fluoxetine label notes that symptoms of sexual dysfunction occasionally persist after the drug is discontinued.
• Sleep changes: insomnia or, in some patients, drowsiness
• Headache, dizziness, dry mouth, and weight changes
• Increased anxiety or jitteriness in the first weeks of treatment
• Serotonin syndrome (agitation, fast heart rate, high blood pressure, high temperature, tremor, and clonus) when serotonin activity rises too high
• QT interval prolongation, most notably with citalopram. Because of that risk the FDA caps citalopram at 40 mg once daily, and at 20 mg once daily for patients over 60, patients with hepatic impairment, and CYP2C19 poor metabolizers. Escitalopram prolongs the QT interval to a lesser degree and carries no comparable dose cap.

Key interactions

• MAOIs (phenelzine, tranylcypromine, selegiline, isocarboxazid) and linezolid combined with an SSRI can cause life-threatening serotonin syndrome, so they are contraindicated and need a washout period between them (at least 14 days after stopping an MAOI)
• Other serotonergic drugs raise serotonin syndrome risk: triptans (such as sumatriptan), tramadol, other antidepressants (SNRIs and TCAs), St John's wort, dextromethorphan, and lithium
• NSAIDs, aspirin, and anticoagulants (such as warfarin) combined with an SSRI increase the risk of gastrointestinal bleeding
• CYP450 enzyme inhibition varies a lot by drug: fluoxetine and paroxetine are strong CYP2D6 inhibitors, sertraline is a weaker dose-dependent CYP2D6 inhibitor, and citalopram and escitalopram are weak. Fluoxetine and fluvoxamine inhibit CYP2C19, and fluvoxamine also inhibits CYP1A2 and CYP3A4, which can raise levels of other drugs
• Other QT-prolonging drugs add to the QT risk of citalopram and escitalopram, and a CYP2C19 inhibitor such as cimetidine or omeprazole lowers the citalopram maximum to 20 mg once daily

Patient counseling notes

• The full mood benefit can take several weeks, so keep taking the medication even if it does not feel like it is working at first
• Do not stop an SSRI abruptly: sudden stopping can cause discontinuation syndrome (dizziness, flu-like feelings, irritability, and electric-shock sensations), so the dose is tapered down
• Report signs of serotonin syndrome (agitation, fast heartbeat, sweating, shivering, muscle twitching, high fever) right away, especially when adding another serotonergic drug
• Watch for new or worsening thoughts of self-harm or changes in behavior, especially in patients 24 and younger, because of the boxed warning for suicidal thoughts and behaviors. Every patient starting an antidepressant is monitored closely, not only younger ones.
• Tell the pharmacist about all other medicines, since combining serotonergic drugs (triptans, tramadol, MAOIs, St John's wort) can be dangerous
• Most SSRIs are taken once daily (fluvoxamine tablets may be taken once daily at bedtime or twice daily), and if the dose upsets the stomach it may be taken with food
• Paroxetine is generally avoided in pregnancy because of first trimester cardiac malformations, so flag a paroxetine prescription for a patient who is or may become pregnant

Practice questions

Then test the whole class in context with the drug class identification quiz and the brand and generic quiz.

Related drug-class guides

Official sources

• StatPearls: Selective Serotonin Reuptake Inhibitors (NBK554406)
• MedlinePlus: Sertraline (Zoloft)
• MedlinePlus: Escitalopram (Lexapro)
• MedlinePlus: Fluvoxamine (Luvox)
• MedlinePlus: Serotonin syndrome
• DailyMed: ZOLOFT (sertraline) FDA label, boxed warning and serotonin syndrome drug list
• DailyMed: LEXAPRO (escitalopram) FDA label
• DailyMed: PROZAC (fluoxetine) FDA label
• FDA label: CELEXA (citalopram), QT prolongation and maximum daily dose

PTCB Quiz Prep is an independent study resource, not affiliated with the Pharmacy Technician Certification Board. This guide is for exam preparation, not medical advice. See our editorial standards. Part of the PTCB Medications domain.