Antidepressant Drugs

Correct Answer

B — Noradrenergic and specific serotonergic antidepressants

Rationale

Mirtazapine is classified as a noradrenergic and specific serotonergic antidepressant, a class named for its mechanism of enhancing norepinephrine and serotonin output through receptor blockade rather than transporter inhibition. This class label distinguishes mirtazapine from all reuptake-inhibiting antidepressant classes. Selective serotonin reuptake inhibitors include fluoxetine, sertraline, and paroxetine. Serotonin-norepinephrine reuptake inhibitors include venlafaxine and duloxetine. Tricyclic antidepressants include amitriptyline and nortriptyline.

Correct Answer

A — Duloxetine

Rationale

Duloxetine is classified as the preferred serotonin-norepinephrine reuptake inhibitor when analgesia is a co-primary treatment goal. It holds Food and Drug Administration approvals for diabetic peripheral neuropathic pain, fibromyalgia, and chronic musculoskeletal pain in addition to its antidepressant and anxiolytic indications — the broadest pain portfolio among the serotonin-norepinephrine reuptake inhibitors. Venlafaxine is approved for mood and anxiety disorders but carries fewer pain-specific approvals than duloxetine. Desvenlafaxine is approved for major depressive disorder only. Levomilnacipran is approved for major depressive disorder only and has not established the pain indication portfolio that distinguishes duloxetine.

Correct Answer

C — Levomilnacipran

Rationale

Levomilnacipran is classified as the serotonin-norepinephrine reuptake inhibitor with the strongest norepinephrine transporter selectivity — it inhibits the norepinephrine transporter more potently than the serotonin reuptake transporter, giving it a more noradrenergic profile than any other agent in the class. This selectivity profile is a classification-level distinguishing feature of levomilnacipran and underlies its distinctive adverse effect of urinary hesitancy, which is more prominent with levomilnacipran than with duloxetine or venlafaxine. Duloxetine achieves meaningful inhibition of both transporters across its full dose range. Venlafaxine's norepinephrine transporter inhibition is dose-dependent and favors serotonin at lower doses. Desvenlafaxine inhibits both transporters but does not have the norepinephrine-predominant selectivity that characterizes levomilnacipran.

Correct Answer

D — Bupropion

Rationale

Bupropion is the only antidepressant in this group that is also classified as an approved pharmacotherapy for smoking cessation, marketed under the brand name Zyban for this indication. It was the first non-nicotine pharmacotherapy approved for smoking cessation. Mirtazapine, duloxetine, and venlafaxine are approved for psychiatric indications only and have no approved smoking cessation indication. Bupropion approximately doubles quit rates compared to placebo and can be combined with nicotine replacement therapy for additive benefit.

Correct Answer

B — Duloxetine

Rationale

Duloxetine carries a contraindication in narrow-angle glaucoma that is not shared broadly across the serotonin-norepinephrine reuptake inhibitor class. This contraindication is a classification-level safety fact distinguishing duloxetine from venlafaxine, desvenlafaxine, and levomilnacipran as first-line alternatives when narrow-angle glaucoma is present. Venlafaxine, desvenlafaxine, and levomilnacipran do not carry this specific contraindication label within the class.

Correct Answer

A — Mirtazapine

Rationale

Mirtazapine is classified as an antidepressant that does not cause sexual dysfunction. Because it has no serotonin reuptake transporter activity, it does not produce the decreased libido, delayed orgasm, and anorgasmia that affect a substantial proportion of patients on selective serotonin reuptake inhibitors and serotonin-norepinephrine reuptake inhibitors. This property makes mirtazapine a preferred choice in patients for whom sexual function is a treatment priority. Sertraline, duloxetine, and venlafaxine all block the serotonin reuptake transporter and are associated with serotonin-mediated sexual dysfunction.

Correct Answer

A — Blockade of presynaptic alpha-2 adrenergic autoreceptors removes the inhibitory brake on norepinephrine and serotonin release

Rationale

Presynaptic alpha-2 adrenergic autoreceptors act as a negative feedback brake: when norepinephrine activates them, they suppress further norepinephrine release from the noradrenergic neuron. Alpha-2 receptors located on serotonergic terminals similarly suppress serotonin release when activated by norepinephrine. Mirtazapine blocks both sets of alpha-2 receptors, removing the inhibitory brake and allowing greater norepinephrine and serotonin output with each neuronal firing — a presynaptic disinhibition mechanism entirely distinct from transporter blockade. Monoamine oxidase inhibition describes the mechanism of phenelzine and tranylcypromine. Serotonin-1A agonism describes the mechanism of vilazodone and buspirone. Dopamine transporter blockade describes the mechanism of bupropion.

Correct Answer

C — Serotonin-3 receptor blockade in the gastrointestinal tract and chemoreceptor trigger zone

Rationale

Serotonin-3 receptors in the gastrointestinal tract and brainstem chemoreceptor trigger zone mediate the nausea and vomiting response to elevated serotonin levels. When selective serotonin reuptake inhibitors and serotonin-norepinephrine reuptake inhibitors raise synaptic serotonin, activation of these serotonin-3 receptors produces the nausea that is the most common early adverse effect of those classes. Mirtazapine blocks serotonin-3 receptors postsynaptically, preventing this activation and producing a pronounced antiemetic effect. This same mechanism underlies the antiemetic activity of ondansetron. Serotonin-2A blockade by mirtazapine produces other effects, including elimination of sleep disturbances, but does not account for the antiemetic property. Alpha-2 blockade increases monoamine release and does not mediate the antiemetic effect.

Correct Answer

B — Histamine H1 blockade, which drives sedation, is near-maximal across the full dose range, while higher doses produce greater noradrenergic activation that partially counteracts the sedation

Rationale

Mirtazapine's histamine H1 antagonism is near-maximal even at 15 mg and does not increase substantially with dose escalation. Sedation from H1 blockade is therefore similar at 15 mg and 30 to 45 mg. However, as the dose increases, alpha-2 receptor blockade drives progressively greater norepinephrine release, and noradrenergic activation partially counteracts the histaminergic sedation. The net result is that patients may actually feel less sedated at 30 mg than at 15 mg — the opposite of the usual expectation for dose escalation. Clinically, this means patients troubled by daytime sedation at 15 mg may tolerate higher doses better, and mirtazapine is most commonly dosed at 30 to 45 mg for antidepressant efficacy with less daytime sedation than the starting dose produces.

Correct Answer

D — Patients with pre-existing liver disease or substantial alcohol use

Rationale

Duloxetine carries a hepatotoxicity risk — rare but reported — that is not shared broadly across the serotonin-norepinephrine reuptake inhibitor class. It should be avoided in patients with pre-existing liver disease or substantial alcohol use, both of which increase susceptibility to hepatic injury. This contraindication distinguishes duloxetine from venlafaxine, desvenlafaxine, and levomilnacipran, which do not carry this specific hepatotoxicity warning. Seizure disorder and eating disorders are absolute contraindications for bupropion, not duloxetine. Poorly controlled hypertension is a relative caution for the entire serotonin-norepinephrine reuptake inhibitor class due to noradrenergic blood pressure effects, but is not a duloxetine-specific contraindication based on hepatotoxicity.

Correct Answer

A — At low doses venlafaxine primarily blocks the serotonin reuptake transporter; at higher doses meaningful norepinephrine transporter inhibition emerges, producing a dual mechanism the patient has not yet received

Rationale

Venlafaxine has a dose-dependent pharmacological profile. At low doses of 75 mg per day or below, it behaves pharmacologically like a selective serotonin reuptake inhibitor, with the serotonin reuptake transporter as its primary target and minimal norepinephrine transporter inhibition. At 150 mg per day and above, meaningful norepinephrine transporter inhibition emerges, and at 225 mg per day the dual serotonin and norepinephrine mechanism is fully expressed. A patient who has not responded at 75 mg has received a serotonin-predominant trial but has not yet been exposed to the full dual mechanism — escalation is therefore pharmacologically rational before concluding that venlafaxine has failed. Venlafaxine does not meaningfully inhibit the dopamine transporter at any clinical dose. Its blood-brain barrier penetration is not dose-limited in the clinical range.

Correct Answer

C — The immediate-release formulation has a short half-life requiring multiple daily doses and producing larger inter-dose concentration fluctuations that increase discontinuation syndrome risk from missed doses

Rationale

Venlafaxine immediate-release has a short half-life and must be taken two to three times daily to maintain adequate plasma concentrations. This dosing schedule creates larger inter-dose fluctuations in plasma levels, and missed doses can produce a rapid drop in serotonin reuptake transporter occupancy sufficient to trigger discontinuation syndrome symptoms within 24 hours. The extended-release formulation smooths plasma concentration curves, permits once-daily dosing, and substantially reduces — though does not eliminate — discontinuation syndrome risk from missed or delayed doses. Venlafaxine does not have a bioavailability problem from first-pass metabolism that the extended-release formulation corrects, and both formulations share the same cytochrome P450 interaction profile since they contain the same active compound.

Correct Answer

B — Patients with bulimia nervosa or a prior seizure disorder

Rationale

Both bulimia nervosa and prior seizure disorder are absolute contraindications to bupropion due to seizure risk. Patients with a prior seizure disorder already have a lowered seizure threshold, and adding bupropion further reduces it. Bulimia nervosa with purging produces electrolyte abnormalities — particularly hypokalemia and hypomagnesemia — that independently lower the seizure threshold and interact synergistically with bupropion's proconvulsant effect. Anorexia nervosa with purging carries the same contraindication. Poorly controlled hypertension is a caution for serotonin-norepinephrine reuptake inhibitors due to noradrenergic blood pressure elevation, not a bupropion-specific seizure contraindication. Narrow-angle glaucoma is a contraindication specific to duloxetine. Pre-existing liver disease is the contraindication specific to duloxetine's hepatotoxicity risk.

Correct Answer

D — It raises basal dopamine tone in the nucleus accumbens, attenuating the drop in dopamine that drives nicotine craving during withdrawal

Rationale

Nicotine dependence is mediated in part through dopaminergic reward pathways in the nucleus accumbens — nicotine raises dopamine release in this region, and withdrawal produces a drop in dopamine tone that drives craving. Bupropion blocks the dopamine reuptake transporter, raising basal dopamine levels in the nucleus accumbens and attenuating the dopamine deficit of nicotine withdrawal, which reduces craving. Bupropion also weakly blocks nicotinic acetylcholine receptors, which may reduce cigarette reinforcement if smoking occurs during treatment — but this peripheral mechanism is secondary to the dopaminergic action in the nucleus accumbens. Bupropion has no meaningful serotonergic activity and does not inhibit monoamine oxidase.

Correct Answer

D — Duloxetine

Rationale

Duloxetine is the appropriate choice because it is the only antidepressant among the options with a Food and Drug Administration approval specifically for diabetic peripheral neuropathic pain, in addition to its antidepressant indication. Its dual blockade of the serotonin reuptake transporter and the norepinephrine transporter augments descending noradrenergic pain modulation in the spinal cord, reducing nociceptive transmission at the dorsal horn — the mechanism underlying its analgesic efficacy. Fluoxetine blocks only the serotonin reuptake transporter and does not reliably activate descending noradrenergic pain pathways at therapeutic doses; it has no approved pain indication. Mirtazapine increases norepinephrine and serotonin output through alpha-2 blockade but does not carry pain-specific approvals. Bupropion acts on dopamine and norepinephrine transporters but has no approved indication for neuropathic pain.

Correct Answer

A — Serotonin-3 receptor blockade eliminates nausea; serotonin-2C receptor blockade stimulates appetite

Rationale

Mirtazapine blocks postsynaptic serotonin receptors at multiple subtypes. Serotonin-3 receptor blockade in the gastrointestinal tract and brainstem chemoreceptor trigger zone produces a pronounced antiemetic effect, explaining why patients on mirtazapine do not experience the nausea that commonly occurs when serotonin levels rise with selective serotonin reuptake inhibitors or serotonin-norepinephrine reuptake inhibitors. Serotonin-2C receptor blockade in the hypothalamus disinhibits appetite regulation, stimulating food intake and promoting weight gain. Together these two postsynaptic serotonin receptor actions explain the observed clinical findings. Weight gain from mirtazapine is among the most pronounced of any antidepressant and should be discussed with patients before initiating treatment. Histamine H1 blockade also contributes to weight gain but through sedation-related mechanisms and metabolic effects, not primarily through appetite stimulation.

Correct Answer

C — Bupropion, because purging-related electrolyte abnormalities lower the seizure threshold and interact synergistically with bupropion's proconvulsant effect

Rationale

Bupropion is absolutely contraindicated in bulimia nervosa and anorexia nervosa with purging. Purging behavior causes electrolyte abnormalities — particularly hypokalemia and hypomagnesemia — that independently lower the seizure threshold. When combined with bupropion, which also lowers the seizure threshold in a dose-dependent manner, the two mechanisms interact synergistically to place these patients at substantially elevated risk of seizure. This contraindication is among the most high-yield bupropion facts for Step 1 because it differs entirely from the contraindications of all other antidepressant classes. Mirtazapine, duloxetine, and venlafaxine do not carry this seizure-based contraindication and are acceptable options in patients with eating disorders when clinically indicated.

Correct Answer

B — Venlafaxine blocks the norepinephrine transporter in addition to the serotonin reuptake transporter, increasing sympathetic tone and raising blood pressure in a way that sertraline's serotonin-only mechanism does not

Rationale

Venlafaxine is a serotonin-norepinephrine reuptake inhibitor — it blocks both the serotonin reuptake transporter and the norepinephrine transporter. Norepinephrine reuptake blockade increases norepinephrine availability at adrenergic synapses throughout the cardiovascular system, raising sympathetic tone and producing dose-dependent increases in blood pressure and heart rate. Sertraline, a selective serotonin reuptake inhibitor, blocks only the serotonin reuptake transporter and does not substantially raise norepinephrine levels, which is why it did not affect this patient's blood pressure. Blood pressure monitoring is recommended at baseline, at each dose increase, and periodically during venlafaxine treatment. Venlafaxine does not directly stimulate alpha-1 receptors, does not raise aldosterone levels through serotonergic mechanisms, and does not inhibit cytochrome P450 2D6 to a clinically consequential degree.