Selective Serotonin
Reuptake Inhibitors — Compared

Fluoxetine was the first SSRI approved in the United States (1987) and remains widely used. Its defining characteristic is an extraordinarily long half-life — not just because the parent drug persists for days, but because it produces an active metabolite (norfluoxetine) that stays active in the body for weeks.

This makes fluoxetine uniquely forgiving: patients who occasionally miss a dose are unlikely to notice any effect, and the long metabolite effectively acts as a built-in slow taper when the drug is stopped. Discontinuation syndrome is rare — the drug does the gradual step-down on its own.

Fluoxetine tends to be activating rather than sedating. This can be helpful for patients with fatigue and low energy, but may worsen insomnia or anxiety — particularly early in treatment. It is a moderate inhibitor of the CYP2D6 enzyme, creating drug interaction potential with certain other medications.

Sertraline is the most commonly prescribed SSRI in the world and is often the first antidepressant a prescriber reaches for — not because it is dramatically more effective than alternatives, but because it has the broadest FDA approval set (six indications), a favorable interaction profile, and decades of real-world tolerability data.

Its half-life of around 26 hours supports once-daily dosing with a relatively predictable pharmacokinetic profile. It is a weak inhibitor of CYP2D6, which means fewer drug-drug interactions than paroxetine while still warranting attention when combined with other CYP2D6-sensitive drugs.

Sertraline is considered reasonably well tolerated. GI side effects — particularly nausea and loose stools — are among the most commonly reported, especially early in treatment, and typically improve within the first few weeks.

Escitalopram is the most chemically selective of all SSRIs. Citalopram (its parent compound) is a racemic mixture of two mirror-image molecules; escitalopram isolates just the S-enantiomer — the one responsible for nearly all SSRI activity — and discards the R-enantiomer, which contributes mainly to side effects.

This higher selectivity translates to fewer off-target receptor interactions and, in clinical practice, a side effect profile that many patients find cleaner than other SSRIs. It has minimal effect on cytochrome P450 enzymes, making it one of the better choices when a patient is already on multiple medications.

Its FDA approvals are narrower than sertraline (only depression and generalized anxiety disorder), but prescribers commonly use it off-label for other anxiety conditions given its profile. It is frequently considered a first-line option alongside sertraline.

Citalopram is the parent compound from which escitalopram was derived. As a racemic mixture of the active S-enantiomer and the R-enantiomer, it requires higher amounts to achieve the same SERT activity as escitalopram — and the R-enantiomer contributes some off-target effects, including a notable impact on cardiac ion channels.

At higher amounts, citalopram can prolong the QTc interval on an ECG — a measure of cardiac repolarization — in a dose-dependent way. The FDA issued a safety communication on this, and prescribers cap the upper strength at 40mg. In elderly patients, those with liver disease, and those taking other QTc-prolonging medications, 20mg is generally the recommended ceiling.

Despite this restriction, citalopram is widely used and well tolerated within its safe range. Its long half-life (~35 hours) supports stable once-daily blood levels.

Paroxetine is the most pharmacologically complicated SSRI. It has the shortest effective half-life among the class, the strongest anticholinergic profile of any SSRI, and is the most potent inhibitor of CYP2D6 — an enzyme responsible for metabolizing dozens of commonly used drugs, including beta-blockers, opioids, and antipsychotics.

Its anticholinergic activity (blocking muscarinic receptors) produces a distinct side effect signature: dry mouth, constipation, urinary hesitancy, blurred vision, and weight gain that tends to be more pronounced than other SSRIs. These effects can be problematic for elderly patients in particular.

The short half-life, combined with no significant active metabolites, means that when paroxetine is reduced or stopped, serotonin levels in the synapse fall rapidly. This makes discontinuation syndrome particularly pronounced — the "brain zaps," flu-like symptoms, and irritability associated with stopping SSRIs are most severe with paroxetine.

Fluvoxamine is rarely used as a first-line antidepressant today, but it remains the most commonly prescribed SSRI for OCD in some countries and the only SSRI with a dedicated OCD formulation for children aged 8 and older in the United States.

Its pharmacology is more complex than other SSRIs because it has particularly strong sigma-1 receptor agonist activity — a receptor involved in inflammation regulation, endoplasmic reticulum stress, and immune signaling. This property sparked interest in fluvoxamine as a potential COVID-19 treatment during the pandemic; research in this area is ongoing.

The drug interaction profile is the most significant clinical consideration. Fluvoxamine is a strong inhibitor of both CYP1A2 and CYP3A4, affecting the metabolism of theophylline, warfarin, clozapine, ramelteon, and dozens of other medications. Patients on complex medication regimens require careful review before starting.

Vilazodone stands apart from other SSRIs by adding a second mechanism: it is a partial agonist at the 5-HT1A serotonin receptor. This receptor is found on both presynaptic neurons (where it acts as an autoreceptor, modulating serotonin release) and postsynaptic neurons. Partial agonism at this receptor was the strategy behind the antidepressant buspirone, and vilazodone effectively combines that approach with SSRI activity in a single molecule.

The clinical benefit of this dual mechanism remains a matter of ongoing debate. Manufacturer-funded studies suggested fewer sexual side effects compared to other SSRIs, but independent real-world comparisons have been less conclusive. Sexual dysfunction is still a reported side effect.

The most important practical point about vilazodone is its food requirement: bioavailability drops by approximately 50% when taken on an empty stomach. It must be taken with a meal to achieve consistent blood levels.