Antidepressant Drugs

Correct Answer

B — Fluoxetine

Rationale

Fluoxetine is classified as a potent inhibitor of cytochrome P450 2D6, one of the most clinically consequential enzyme inhibition profiles among the selective serotonin reuptake inhibitors. Paroxetine shares this classification. Escitalopram has minimal inhibition of all major cytochrome P450 isoforms and is the preferred choice when drug interactions are a primary concern. Sertraline has only weak cytochrome P450 2D6 inhibition at standard doses. Fluvoxamine is a potent inhibitor of cytochrome P450 1A2, not cytochrome P450 2D6, and is classified separately by its isoform specificity.

Correct Answer

A — Paroxetine

Rationale

Paroxetine carries the highest discontinuation syndrome risk of any selective serotonin reuptake inhibitor. It has the shortest half-life in the class and produces no active metabolites, meaning plasma concentrations fall rapidly after the last dose. Fluoxetine carries the lowest discontinuation syndrome risk because its active metabolite norfluoxetine has a half-life of one to two weeks and provides intrinsic self-tapering after the drug is stopped. Sertraline and escitalopram carry intermediate discontinuation syndrome risk and are generally well tolerated with gradual tapering.

Correct Answer

D — Fluvoxamine

Rationale

Fluvoxamine is classified as the most potent cytochrome P450 1A2 inhibitor among the selective serotonin reuptake inhibitors. This distinguishes its interaction profile from paroxetine and fluoxetine, which are potent cytochrome P450 2D6 inhibitors. Citalopram has minimal inhibition of all major cytochrome P450 isoforms. The cytochrome P450 1A2 inhibition of fluvoxamine has high-stakes clinical consequences for co-administered drugs with narrow therapeutic windows that depend on this enzyme for clearance, including clozapine, theophylline, and tizanidine.

Correct Answer

C — Sertraline

Rationale

Sertraline is classified as the preferred selective serotonin reuptake inhibitor in pregnancy. It is among the most extensively studied antidepressants during pregnancy, has a clean drug interaction profile, and carries evidence of cardiac safety. Paroxetine is the selective serotonin reuptake inhibitor most avoided in pregnancy due to signals of cardiovascular malformation in neonates with first-trimester exposure. Fluvoxamine has a high drug interaction burden and limited pregnancy data compared to sertraline. Citalopram carries a QTc prolongation concern at higher doses and is not the preferred choice in pregnancy.

Correct Answer

A — Citalopram

Rationale

Citalopram carries the most significant QTc prolongation concern of the selective serotonin reuptake inhibitors and is subject to a Food and Drug Administration dose cap of 40 mg per day in most patients and 20 mg per day in patients over age 60, those with hepatic impairment, and cytochrome P450 2C19 poor metabolizers. Escitalopram, the S-enantiomer of citalopram, carries a lesser but real QTc prolongation risk with its own dose limitation in high-risk patients. Fluoxetine and paroxetine are classified by their potent cytochrome P450 2D6 inhibition, not QTc prolongation. Fluvoxamine is classified by its cytochrome P450 1A2 inhibition.

Correct Answer

C — Fluoxetine

Rationale

Fluoxetine requires a five-week washout period before a monoamine oxidase inhibitor can be started, compared to the standard two-week washout required after stopping any other selective serotonin reuptake inhibitor. This extended washout distinguishes fluoxetine from every other agent in the class and is a classification-level fact every prescriber must know before switching between these drug classes. Sertraline, paroxetine, escitalopram, citalopram, and fluvoxamine all require only a two-week washout before monoamine oxidase inhibitor initiation.

Correct Answer

C — Neuromuscular findings, autonomic instability, and altered mental status

Rationale

Serotonin syndrome produces a triad of neuromuscular findings, autonomic instability, and altered mental status. The neuromuscular findings are the most diagnostically distinctive and include clonus — rhythmic involuntary muscle contractions, particularly at the ankles and eyes — along with hyperreflexia, myoclonus, and tremor. Autonomic instability produces hyperthermia, diaphoresis, tachycardia, hypertension, and diarrhea. Altered mental status ranges from agitation and anxiety to confusion and delirium. The presence of all three elements simultaneously is not required for diagnosis. Bradycardia and lead-pipe rigidity characterize neuroleptic malignant syndrome. Miosis, respiratory depression, and decreased bowel sounds describe opioid toxidrome. Anticholinergic syndrome produces dry flushed skin and urinary retention without the neuromuscular excitability of serotonin syndrome.

Correct Answer

B — Clonus and hyperreflexia

Rationale

Clonus — particularly ocular clonus — and hyperreflexia are the neuromuscular findings that most reliably distinguish serotonin syndrome from neuroleptic malignant syndrome. Serotonin syndrome reflects excess serotonergic excitation, which drives neuromuscular hyperactivity: clonus, hyperreflexia, myoclonus, and tremor. Neuroleptic malignant syndrome reflects dopamine receptor blockade and produces lead-pipe rigidity and bradyreflexia — the opposite neuromuscular picture. This distinction determines treatment: serotonin syndrome is treated with cyproheptadine and benzodiazepines, while neuroleptic malignant syndrome is treated with bromocriptine and dantrolene. Applying the treatment of one condition to the other provides no benefit and delays appropriate care. Flaccid paralysis and resting pill-rolling tremor are features of other neurological conditions, not of either toxidrome.

Correct Answer

A — Clinically significant muscarinic receptor antagonism not shared by other selective serotonin reuptake inhibitors

Rationale

Paroxetine has clinically significant muscarinic acetylcholine receptor antagonism that is unique among the selective serotonin reuptake inhibitors at therapeutic doses. Blockade of muscarinic receptors reduces parasympathetic tone, producing dry mouth from reduced salivary secretion, urinary hesitancy from reduced detrusor contraction, and constipation from reduced gastrointestinal motility — the hallmarks of anticholinergic adverse effects. This property is not shared by sertraline, fluoxetine, escitalopram, citalopram, or fluvoxamine, which is why these adverse effects are specific to paroxetine within the class. Histamine H1 blockade produces sedation and weight gain but does not account for the anticholinergic pattern. Alpha-1 adrenergic antagonism produces orthostatic hypotension. Excess serotonergic stimulation produces nausea, diarrhea, and sexual dysfunction — not dry mouth or urinary hesitancy.

Correct Answer

D — They inhibit cytochrome P450 2D6, reducing conversion of tamoxifen to its active metabolite endoxifen

Rationale

Tamoxifen is converted to endoxifen, its pharmacologically active metabolite, primarily by cytochrome P450 2D6. Paroxetine and fluoxetine are potent inhibitors of cytochrome P450 2D6 and substantially reduce endoxifen production when co-administered with tamoxifen. Because endoxifen mediates the anticancer efficacy of tamoxifen, reduced endoxifen levels may diminish the breast cancer survival benefit. For patients requiring both an antidepressant and tamoxifen, preferred alternatives include venlafaxine, citalopram, or escitalopram, which have minimal cytochrome P450 2D6 inhibitory activity. Inhibiting renal clearance, competing for receptor binding, and inducing cytochrome P450 2D6 each describe different mechanisms and do not apply to this interaction.

Correct Answer

C — Fluoxetine has an active metabolite, norfluoxetine, with a half-life of one to two weeks that provides intrinsic self-tapering after the drug is stopped

Rationale

Fluoxetine is metabolized to norfluoxetine, an active metabolite with a half-life of one to two weeks. When fluoxetine is stopped, norfluoxetine continues to occupy the serotonin reuptake transporter for weeks, producing a gradual self-directed taper. This intrinsic tapering effect is why abrupt discontinuation of fluoxetine rarely produces the flu-like symptoms, electric-shock sensations, and irritability that characterize discontinuation syndrome with shorter-acting agents. Paroxetine, which has the shortest half-life and no active metabolites in the class, carries the highest discontinuation syndrome risk for the opposite reason. Gut wall accumulation, protein binding release kinetics, and adipose tissue sequestration do not account for the clinical profile of fluoxetine after discontinuation.

Correct Answer

B — Sexual dysfunction, including decreased libido, delayed orgasm, and anorgasmia

Rationale

Sexual dysfunction is the most clinically persistent and treatment-limiting adverse effect of the selective serotonin reuptake inhibitor class. Unlike gastrointestinal adverse effects, which typically attenuate over the first one to two weeks as tolerance develops, sexual dysfunction does not reliably improve with continued treatment and often persists for the duration of therapy — making it a leading cause of antidepressant nonadherence. Nausea and gastrointestinal upset are the most common early adverse effects but generally improve with time and can be reduced by taking the medication with food. Insomnia and activation are early effects, most prominent with fluoxetine, and tend to diminish over time. Hyponatremia is a class effect that is relevant primarily in elderly patients and occurs less commonly across the broader patient population.

Correct Answer

D — Fluvoxamine inhibits cytochrome P450 1A2, reducing clozapine clearance and raising its plasma concentrations to potentially toxic levels

Rationale

Clozapine is metabolized primarily by cytochrome P450 1A2. Fluvoxamine is the most potent cytochrome P450 1A2 inhibitor among the selective serotonin reuptake inhibitors and can raise clozapine plasma concentrations dramatically when the two are co-administered. Elevated clozapine levels increase the risk of seizures, sedation, and other dose-dependent toxicities. This interaction is of sufficient clinical concern that fluvoxamine is generally avoided in patients receiving clozapine. Fluvoxamine inhibits cytochrome P450 1A2 — it does not induce it. The interaction is pharmacokinetic, not pharmacodynamic, and does not involve receptor competition. Cytochrome P450 2D6 inhibition describes the profile of paroxetine and fluoxetine, not fluvoxamine.

Correct Answer

A — Blocking serotonin reuptake while simultaneously preventing its enzymatic breakdown eliminates both clearance mechanisms, causing dangerous serotonin accumulation at synapses

Rationale

Serotonin is normally cleared from the synapse by two mechanisms: reuptake into the presynaptic neuron via the serotonin reuptake transporter, and enzymatic degradation by monoamine oxidase inside the neuron. A selective serotonin reuptake inhibitor blocks reuptake, and an irreversible monoamine oxidase inhibitor blocks enzymatic degradation — simultaneously eliminating both clearance pathways. The result is profound serotonin accumulation at central and peripheral synapses, producing the clinical triad of serotonin syndrome: neuromuscular excitation, autonomic instability, and altered mental status. This combination is absolutely contraindicated and has caused fatalities. The two drug classes do not compete at the transporter, do not affect each other's plasma concentrations through this mechanism, and do not directly stimulate postsynaptic receptors.

Correct Answer

D — Residual sertraline continued blocking serotonin reuptake while phenelzine blocked monoamine oxidase, eliminating both serotonin clearance mechanisms and causing serotonin accumulation

Rationale

After discontinuation, sertraline remains pharmacologically active for days as plasma concentrations decline. When phenelzine was started without a two-week washout, residual sertraline was still blocking the serotonin reuptake transporter while phenelzine simultaneously blocked monoamine oxidase — the two serotonin clearance mechanisms eliminated together. The resulting serotonin accumulation produced serotonin syndrome, characterized by its triad: neuromuscular findings (clonus), autonomic instability (diaphoresis, hyperthermia, tachycardia), and altered mental status (agitation). Management requires stopping both agents immediately. Phenelzine is a monoamine oxidase inhibitor, not a dopamine antagonist, and does not cause neuroleptic malignant syndrome.

Correct Answer

A — Venlafaxine

Rationale

Tamoxifen is converted to endoxifen by cytochrome P450 2D6, and endoxifen mediates the drug's anticancer effect. Antidepressants that potently inhibit cytochrome P450 2D6 reduce endoxifen production and may compromise breast cancer outcomes. Venlafaxine, a serotonin-norepinephrine reuptake inhibitor, has minimal cytochrome P450 2D6 inhibitory activity and is a preferred antidepressant choice in patients receiving tamoxifen. Fluoxetine and paroxetine are both potent cytochrome P450 2D6 inhibitors and should be avoided in this setting. Fluvoxamine is a potent cytochrome P450 1A2 inhibitor with a high drug interaction burden and is not the preferred choice here.

Correct Answer

C — Paroxetine has the shortest half-life and no active metabolites in the class, causing rapid loss of serotonin reuptake transporter occupancy after the last dose

Rationale

Paroxetine has the shortest effective half-life and produces no active metabolites, so plasma concentrations and serotonin reuptake transporter occupancy fall rapidly after the last dose. This abrupt loss of serotonergic tone produces discontinuation syndrome — a physiological response to sudden serotonin withdrawal that manifests as flu-like symptoms, insomnia, nausea, and sensory disturbances described as electric shocks or brain zaps. Fluoxetine, by contrast, carries the lowest discontinuation syndrome risk in the class because its active metabolite norfluoxetine provides weeks of residual serotonin reuptake transporter occupancy after the last dose. The syndrome is not caused by changes in paroxetine's own metabolism after stopping, by tissue release kinetics, or by differential receptor affinity.

Correct Answer

B — Paroxetine has clinically significant muscarinic receptor antagonism not shared by other selective serotonin reuptake inhibitors, producing anticholinergic adverse effects

Rationale

Paroxetine is unique among the selective serotonin reuptake inhibitors in having clinically significant muscarinic acetylcholine receptor antagonism at therapeutic doses. Blockade of muscarinic receptors reduces parasympathetic tone, producing dry mouth from reduced salivary secretion, urinary hesitancy from reduced detrusor contraction, and constipation from reduced gastrointestinal motility. These anticholinergic adverse effects are of particular concern in elderly male patients with prostatic enlargement, in whom urinary retention may become a clinical emergency. No other selective serotonin reuptake inhibitor produces this adverse effect profile at standard therapeutic doses. Cytochrome P450 2D6 inhibition does not affect acetylcholine metabolism. Alpha-1 adrenergic blockade and serotonin-3 stimulation produce different adverse effect patterns.