SSRI vs. SNRI

How They Work

SSRIs

• Block reuptake of serotonin in the synapse, leaving more serotonin available
• "Selective" because they primarily target serotonin only
• Effects on mood, anxiety, and obsessions emerge over 2–6 weeks
• Mechanism beyond serotonin elevation is incompletely understood; downstream changes in neuroplasticity likely matter

SNRIs

• Block reuptake of both serotonin AND norepinephrine
• Norepinephrine is involved in arousal, attention, and pain modulation
• The dual action explains both their additional clinical uses (pain, fatigue) and their side-effect profile

Medications in Each Class

SSRIs (Common U.S.)

• Sertraline (Zoloft): broad-spectrum; commonly first-choice
• Escitalopram (Lexapro): typically best-tolerated; favored for generalized anxiety
• Citalopram (Celexa): similar to escitalopram; QT-interval risk at higher doses
• Fluoxetine (Prozac): longest half-life; less withdrawal; activating; FDA-approved in pediatrics
• Paroxetine (Paxil): more sedating; significant weight gain and withdrawal risk
• Fluvoxamine (Luvox): primarily used for OCD
• Vortioxetine (Trintellix): SSRI-plus with multimodal action; often considered separately

SNRIs

• Venlafaxine (Effexor): mostly serotonergic at low doses; norepinephrine effects emerge at higher doses; notorious for severe withdrawal
• Desvenlafaxine (Pristiq): active metabolite of venlafaxine; similar profile
• Duloxetine (Cymbalta): well-balanced serotonin/norepinephrine effects across doses; FDA-approved for chronic pain conditions
• Levomilnacipran (Fetzima): more norepinephrine-selective; less commonly used
• Milnacipran (Savella): in U.S., approved only for fibromyalgia

Side by Side

Conditions Each Treats Best

SSRIs

• Major depressive disorder (first-line)
• Generalized anxiety disorder
• Social anxiety disorder
• Panic disorder
• OCD (often higher doses required)
• PTSD (sertraline and paroxetine FDA-approved)
• Premenstrual dysphoric disorder (PMDD)
• Bulimia nervosa (fluoxetine)
• Pediatric depression and anxiety (fluoxetine, escitalopram)

SNRIs

• Major depression — particularly when SSRI hasn't fully worked
• Generalized anxiety disorder (venlafaxine, duloxetine)
• Diabetic peripheral neuropathy (duloxetine)
• Fibromyalgia (duloxetine, milnacipran)
• Chronic musculoskeletal pain (duloxetine)
• Stress urinary incontinence (duloxetine in some countries)
• Hot flashes (low-dose venlafaxine, sometimes used for menopausal symptoms when hormones aren't an option)
• Severe melancholic depression (some evidence of slight superiority)

Side Effects

Shared by Both Classes

• Nausea (often improves over 1–2 weeks)
• Sexual dysfunction (decreased libido, delayed orgasm) — most common reason for discontinuation
• Sleep disturbance (insomnia or sedation depending on drug)
• Headache
• Initial increase in anxiety in first 1–2 weeks
• Emotional blunting
• Increased suicidal thoughts in young people (FDA black-box warning under age 25)
• Risk of serotonin syndrome with other serotonergic drugs

More Common with SNRIs

• Elevated blood pressure (especially venlafaxine, dose-dependent)
• Sweating
• Increased heart rate
• Activation, jitteriness
• Stronger withdrawal symptoms

Drug-Specific Notes

• Paroxetine: highest weight gain, worst withdrawal, more anticholinergic effects
• Fluoxetine: activating, can worsen insomnia; long half-life means easier discontinuation
• Venlafaxine: notorious for severe withdrawal even with missed dose
• Duloxetine: hepatotoxicity risk, especially with alcohol

Withdrawal and Discontinuation

Both SSRIs and SNRIs can cause discontinuation syndrome — a constellation of dizziness, "brain zaps," nausea, anxiety, irritability, and flu-like symptoms when stopped abruptly. SNRIs (especially venlafaxine) tend to produce more severe withdrawal because of their shorter half-lives and dual mechanism.

Best Practices

• Always taper slowly under medical supervision
• For severe withdrawal, "hyperbolic tapering" (very small reductions, especially at low doses) is increasingly recommended
• Switching to fluoxetine (long half-life) before tapering can ease difficult discontinuations
• Discontinuation symptoms are not the same as relapse — they emerge within days of dose change and resolve with dose increase

The increasing recognition of difficult antidepressant withdrawal — particularly after long-term use — is reshaping prescribing practice toward more cautious initiation and more careful tapering.

How Doctors Choose

Start with SSRI Usually Because

• Generally fewer side effects
• Easier discontinuation
• Lower blood pressure risk
• Strong evidence base across many conditions

Choose SNRI When

• Co-existing chronic pain (especially neuropathic pain or fibromyalgia)
• Prior SSRI trial inadequate
• Severe melancholic depression with prominent fatigue
• Hot flashes during menopause when hormones not feasible

Patient Factors Matter

• Past response (personal or family) is the best predictor
• Side effect priorities: weight, sexual function, sedation, activation
• Other medications and medical conditions
• Cost and formulary coverage

Switching Between Them

• Cross-titration (overlap) often used
• Direct switch sometimes possible at equivalent doses
• Wait periods needed if switching to or from MAOIs
• Always under prescriber supervision
• 4–6 weeks at therapeutic dose needed to assess response on the new medication