Antidepressants: Mechanisms, Classes, Side Effects, and Safe Use

How Antidepressants Work

Antidepressants are medications that alleviate depressive symptoms by modulating neurotransmitter systems involved in mood regulation. Most primarily increase synaptic availability of serotonin, norepinephrine, and/or dopamine by inhibiting their reuptake or metabolism. Downstream effects include enhanced neuroplasticity (via BDNF), normalized HPA axis function, and restored circadian rhythms—processes that take weeks, explaining the delayed therapeutic onset.

Major Classes of Antidepressants

1. Selective Serotonin Reuptake Inhibitors (SSRIs)

Examples: Fluoxetine, sertraline, escitalopram, paroxetine, citalopram
Mechanism: Block serotonin transporter (SERT), increasing synaptic serotonin
First-line for most patients due to favorable side-effect profile, safety in overdose, and once-daily dosing

2. Serotonin-Norepinephrine Reuptake Inhibitors (SNRIs)

Examples: Venlafaxine, duloxetine, desvenlafaxine, levomilnacipran
Mechanism: Inhibit reuptake of both serotonin and norepinephrine (dose-dependent)
Advantages: May be more effective for severe depression, comorbid neuropathic pain, fibromyalgia

3. Atypical / Multimodal Antidepressants

• Bupropion (NDRI): Dopamine/norepinephrine reuptake inhibitor; activating, minimal sexual dysfunction, weight-neutral; contraindicated in seizure disorders
• Mirtazapine (NaSSA): Enhances noradrenergic and specific serotonergic transmission; sedating, increases appetite; useful for insomnia/anorexia
• Trazodone: SARI; primarily used at low doses for sleep
• Vortioxetine: Multimodal: SERT inhibition + 5-HT receptor modulation; procognitive effects
• Vilazodone: SPARI: SERT inhibition + 5-HT1A partial agonism

4. Tricyclic Antidepressants (TCAs)

Examples: Amitriptyline, nortriptyline, imipramine, desipramine
Mechanism: Block serotonin and norepinephrine reuptake; also antagonize muscarinic, histaminic, alpha-adrenergic receptors
Use: Second/third-line for treatment-resistant depression; also for neuropathic pain, migraine prophylaxis

5. Monoamine Oxidase Inhibitors (MAOIs)

Examples: Phenelzine, tranylcypromine, isocarboxazid, selegiline (transdermal)
Mechanism: Irreversibly inhibit MAO-A and MAO-B, increasing monoamine levels
Use: Atypical depression, treatment-resistant cases; require tyramine-restricted diet and strict avoidance of serotonergic drugs

Common Side Effects

Class	Frequent Side Effects
SSRIs/SNRIs	Nausea, headache, insomnia/somnolence, sexual dysfunction, weight changes, GI upset, jitteriness
Bupropion	Insomnia, dry mouth, headache, anxiety, seizure risk at high doses
Mirtazapine	Sedation, weight gain, increased appetite, dry mouth
TCAs	Anticholinergic (dry mouth, constipation, urinary retention, blurred vision), orthostatic hypotension, sedation, cardiac conduction effects, weight gain
MAOIs	Orthostatic hypotension, insomnia, weight gain, sexual dysfunction, hypertensive crisis with tyramine

Safe Use Principles

• Start low, go slow; titrate based on response and tolerability
• Allow 4–8 weeks for adequate trial at therapeutic dose
• Monitor for worsening suicidality, especially in young adults (FDA black box warning)
• Assess for serotonin syndrome with serotonergic combinations
• Taper gradually (over weeks–months) to avoid discontinuation syndrome
• Coordinate with psychotherapy for optimal outcomes

Key Takeaway

Antidepressants are effective tools, not "happy pills." The right choice depends on individual symptom profile, comorbidities, side-effect sensitivity, and preferences. Close collaboration with a prescribing clinician ensures safe, personalized treatment.