Vilazodone

Indications

• FDA-approved only for major depressive disorder in adults, with a target dose of 20 or 40 mg once daily taken with food.
• Not approved for any pediatric indication; a large adolescent depression trial failed to separate from placebo on the primary outcome.
• Off-label for generalized anxiety disorder, where two phase III trials showed benefit at 20 and 40 mg/day but did not yield an approval.
• Sometimes chosen off-label when sexual dysfunction has limited SSRI tolerability, though head-to-head superiority remains unproven.
• A reasonable off-label option for anxious depression given the anxiolytic contribution of its 5-HT1A partial agonism.
• Not a first-line agent in most guidelines because of cost, gastrointestinal burden, and the absence of comparative efficacy advantage.

Mechanism of action

• Blocks the serotonin transporter with SSRI-like potency while simultaneously acting as a partial agonist at postsynaptic and somatodendritic 5-HT1A receptors.
• The 5-HT1A partial agonism mimics buspirone augmentation and is intended to shorten the delay to raphe autoreceptor desensitization.
• That dual action theoretically buffers the early anxiety and sexual side effects that follow pure transporter blockade.
• Has no meaningful affinity for norepinephrine or dopamine transporters, or for muscarinic, histaminic, and adrenergic receptors.
• Clinical trials have not demonstrated a faster onset or lower sexual dysfunction rate than SSRIs despite the mechanistic rationale.

Pharmacokinetics

• Bioavailability is about 72 percent when taken with food and falls by roughly 50 percent when fasting, so every dose must accompany a meal.
• Half-life is approximately 25 hours, giving once-daily dosing with steady state reached after about three days at a stable dose.
• Metabolized principally by CYP3A4 with minor contributions from CYP2C19 and CYP2D6, and it produces no clinically active metabolites.
• Strong CYP3A4 inhibitors roughly double exposure, while strong inducers used beyond two weeks can cut concentrations by about half.
• It is 96 to 99 percent protein bound and eliminated mostly in feces, with less than 2 percent excreted unchanged in urine.

Dosing

• Start 10 mg PO once daily with food for seven days, increase to 20 mg/day for seven days, then to 40 mg/day if needed.
• The effective range is 20 to 40 mg/day, and the label maximum is 40 mg/day; skipping the titration steps sharply increases nausea and diarrhea.
• Cap the dose at 20 mg/day when a strong CYP3A4 inhibitor such as ketoconazole, clarithromycin, or ritonavir is co-prescribed.
• With a strong CYP3A4 inducer used for more than 14 days, the dose may be doubled over one to two weeks, not exceeding 80 mg/day.
• Available as 10, 20, and 40 mg tablets and a 10 mg plus 20 mg starter pack designed to enforce the stepwise titration.
• No adjustment is needed for renal impairment or for mild to moderate hepatic impairment; taper gradually rather than stopping abruptly.

Adverse effects

• Diarrhea occurs in roughly 28 percent of patients and nausea in about 23 percent, making gastrointestinal upset the dominant tolerability issue.
• Taking each dose with a substantial meal and titrating slowly are the two interventions that most reliably reduce those symptoms.
• Insomnia, dizziness, dry mouth, and abnormal dreams are common, and vomiting occurs more often than with standard SSRIs.
• Sexual dysfunction rates in trials were low but the comparisons were placebo-controlled, not against an active SSRI comparator.
• Serotonin syndrome, hyponatremia, activation of mania, and increased bleeding risk with NSAIDs or anticoagulants apply as with any SSRI.
• Weight change is minimal in short-term trials, averaging under 1 kg over eight weeks of treatment.

Monitoring

• Confirm at each visit that the patient is taking every dose with food, since fasting administration halves exposure and mimics nonresponse.
• Assess suicidality, activation, and agitation weekly for the first four weeks and after each dose increase, especially under age 25.
• Track response with the PHQ-9 or MADRS every two to four weeks and allow six to eight weeks at 40 mg/day before declaring failure.
• Check serum sodium at baseline and within two to four weeks in older adults or patients taking diuretics.
• Review the medication list for strong CYP3A4 inhibitors or inducers at initiation and whenever any new prescription is added.

Interactions

• Contraindicated with MAOIs and within 14 days of stopping one, and with linezolid or intravenous methylene blue.
• Strong CYP3A4 inhibitors including ketoconazole, clarithromycin, and ritonavir require the dose to be reduced to 20 mg/day.
• Strong CYP3A4 inducers such as carbamazepine, rifampin, phenytoin, and St. John's wort can halve exposure and cause apparent nonresponse.
• Additive serotonin syndrome risk with triptans, tramadol, fentanyl, lithium, and other serotonergic antidepressants.
• Increases bleeding risk in combination with aspirin, NSAIDs, warfarin, and direct oral anticoagulants.

Special populations

• Pregnancy data are limited, so better-characterized agents such as sertraline are preferred when treatment is needed during gestation.
• Lactation data are essentially absent; if a serotonergic agent is required while nursing, choose one with an established infant safety record.
• Not approved in pediatrics, and the adolescent depression program failed to demonstrate efficacy over placebo.
• No dose adjustment is required by age, but older adults are more susceptible to hyponatremia, falls, and gastrointestinal fluid loss.
• No adjustment is needed in renal impairment or in mild to moderate hepatic impairment; severe hepatic impairment has not been studied.

Clinical pearls

• Always with food. Fasting cuts absorption by half and looks exactly like treatment failure.
• Diarrhea and nausea drive dropouts; the 10-20-40 mg stepwise titration exists to limit them.
• Add a 5-HT1A partial agonist to an SSRI and you have the same idea for far less money.