Antidepressants · Anxiolytics
What Are SSRIs?
Selective Serotonin Reuptake Inhibitors are the most widely prescribed antidepressants worldwide. They treat depression, anxiety, OCD, and several other conditions by increasing available serotonin in the brain.
Mechanism
Block the serotonin transporter (SERT), preventing reuptake of serotonin into the presynaptic neuron and increasing serotonin activity at the synapse.
Common Uses
Major depressive disorder, generalized anxiety disorder, panic disorder, OCD, PTSD, social anxiety disorder, premenstrual dysphoric disorder.
Key Risks
Serotonin syndrome (with MAOIs or tramadol), increased suicidal ideation in young adults, sexual dysfunction, discontinuation syndrome if stopped abruptly.
Examples
Sertraline (Zoloft), fluoxetine (Prozac), escitalopram (Lexapro), citalopram (Celexa), paroxetine (Paxil).
How SSRIs Work
Serotonin is a neurotransmitter that plays a central role in mood regulation, sleep, appetite, and emotional processing. After a neuron releases serotonin into the synapse, a protein called the serotonin transporter (SERT) normally pulls it back into the releasing cell for recycling — a process called reuptake.
SSRIs block SERT, which means serotonin stays in the synapse longer and continues signaling to the receiving neuron. Over weeks, this sustained increase in serotonergic activity leads to downstream changes in brain circuits that govern mood and anxiety. SSRIs are "selective" because they primarily target the serotonin transporter and have little effect on norepinephrine or dopamine transporters, which reduces the side-effect burden compared to older antidepressants such as tricyclics or MAOIs.
Full therapeutic effects typically take 2–6 weeks to develop as the brain adapts to increased serotonin signaling, although some patients notice earlier improvements in sleep and anxiety. SSRIs do not cause immediate mood elevation and are not habit-forming.
Common Side Effects Across the Class
Nausea (especially early on)
Headache
Insomnia or drowsiness
Sexual dysfunction (delayed orgasm, reduced libido)
Dry mouth
Diarrhea or loose stools
Increased sweating
Weight changes with long-term use
Important Interactions & Warnings
- MAOIs — Combining SSRIs with monoamine oxidase inhibitors can cause life-threatening serotonin syndrome. Allow at least 14 days between stopping an MAOI and starting an SSRI.
- Tramadol & other serotonergic agents — Increases risk of serotonin syndrome; symptoms include agitation, rapid heart rate, high temperature, and muscle rigidity.
- Anticoagulants (warfarin, aspirin, NSAIDs) — SSRIs reduce platelet aggregation and can increase bleeding risk, especially GI bleeding, when combined with blood thinners.
- Lithium — Increased serotonergic effects; monitor for toxicity signs.
- Black Box Warning — Increased risk of suicidal thinking and behavior in children, adolescents, and young adults during early treatment. Monitor closely.
- Discontinuation syndrome — Do not stop abruptly; taper slowly under medical supervision to avoid flu-like symptoms, electric-shock sensations, and mood disturbances.
Frequently Asked Questions
Are all SSRIs the same?
No. While all SSRIs share the same mechanism, they differ in half-life, drug interactions, and approved uses. For example, fluoxetine has a very long half-life (days to weeks) making it more forgiving if a dose is missed and easier to discontinue. Paroxetine is more sedating and has stronger anticholinergic effects. Sertraline and escitalopram tend to have the best tolerability profiles in clinical studies. Individual response also varies, so one SSRI may work better for a specific person than another.
Can I switch from one SSRI to another?
Yes, switching SSRIs is common when the first one causes intolerable side effects or does not provide adequate relief. Because all SSRIs work through the same mechanism, there is typically no washout period required when switching directly between them (unlike switching to or from an MAOI). Your prescriber will usually taper one while starting the other, or make a direct switch. Never change your regimen without medical guidance.
How long do I need to take an SSRI?
Guidelines generally recommend staying on an SSRI for at least 6–12 months after symptoms resolve to prevent relapse. For recurrent depression or chronic anxiety disorders, longer-term treatment — sometimes indefinitely — may be recommended. Your provider will help weigh the benefits and risks based on your history.
Do SSRIs cause weight gain?
Some patients experience modest weight gain, particularly with long-term use. Paroxetine is most commonly associated with weight gain among SSRIs. Others, like fluoxetine, may initially cause mild weight loss. Short-term changes are usually small. Diet, activity level, and the underlying depression itself also influence weight, making it hard to attribute changes solely to the medication.