Antidepressant Drugs: Tricyclics, SSRIs, and SNRIs

Tricyclic antidepressants (TCAs) block serotonin and norepinephrine reuptake while also inhibiting muscarinic, α-adrenergic, and histaminic H1 receptors. SSRIs predominantly block serotonin reuptake without significantly affecting norepinephrine. This selectivity makes SSRIs first-line agents, while TCAs' nonselective nature causes a larger side effect profile despite their effectiveness for pain syndromes and insomnia.

SSRIs commonly cause headaches, sweating, gastrointestinal effects, and sexual dysfunction. In combination with other serotonergic drugs, they may cause rare but serious side effects like serotonin syndrome and seizures. SSRIs should be used cautiously in young patients because they may worsen depression and induce suicidal tendencies, requiring careful monitoring.

SNRIs inhibit both serotonin and norepinephrine reuptake, causing noradrenergic stimulation-related adverse effects including hypertension, tachycardia, insomnia, anxiety, and agitation. These effects differ from SSRIs because SNRIs' dual mechanism increases norepinephrine activity, whereas SSRIs target serotonin selectively, resulting in distinct side effect profiles.

SSRIs are mainly used to treat obsessive-compulsive disorder, panic disorder, and social anxiety disorder. They are also effective for depression, bulimia nervosa, and other anxiety-related conditions. Their selective serotonin reuptake mechanism makes them suitable for these psychiatric conditions while minimizing noradrenergic side effects.

Tricyclic antidepressants have a narrow therapeutic window, meaning the difference between an effective dose and a toxic dose is small. Plasma-level monitoring ensures therapeutic efficacy while preventing toxicity. Additionally, TCAs require careful discontinuation to avoid cholinergic rebound and flu-like symptoms that occur with abrupt cessation.

SNRIs are used to treat depression, anxiety disorders, pain syndromes, and fibromyalgia. Their dual inhibition of serotonin and norepinephrine reuptake makes them effective for both mood and pain management. Common adverse effects include nausea, headache, sexual dysfunction, dizziness, insomnia, sedation, and constipation, with abrupt discontinuation potentially causing discontinuation syndrome.

Tricyclic antidepressants are first-generation agents that block reuptake transporters and multiple receptors, whereas antidepressant drugs MAOIs and other agents use different mechanisms. TCAs require plasma-level monitoring and have significant drug interactions, particularly with MAOIs. Understanding these distinctions helps clinicians select appropriate agents based on patient factors and comorbidities.