Psychotropic Medications & Neuropharmacology

Selective serotonin reuptake inhibitors (SSRIs: fluoxetine, sertraline, paroxetine, citalopram, escitalopram, fluvoxamine) selectively block the presynaptic serotonin transporter (SERT), offering first-line safety in major depression, generalized anxiety, panic disorder, PTSD, OCD, and bulimia nervosa. Their signature adverse effects stem from peripheral and central serotonin stimulation: GI nausea/diarrhea (5-HT₃), anorgasmia and decreased libido (30–40%, mediated by 5-HT_2A/2C inhibition of mesolimbic dopamine), SIADH hyponatremia in older adults, impaired platelet aggregation (depletion of platelet 5-HT stores increasing GI bleeding risk with NSAIDs), and dose-dependent QT prolongation with citalopram. Paroxetine has the shortest half-life and strongest antimuscarinic activity, causing the most severe antidepressant discontinuation syndrome (dizziness, "electric shock" paresthesias, flu-like malaise upon abrupt cessation), whereas fluoxetine's active metabolite norfluoxetine has a 7–15 day half-life that auto-tapers. Serotonin-norepinephrine reuptake inhibitors (SNRIs: venlafaxine, desvenlafaxine, duloxetine, milnacipran) inhibit both SERT and NET; because descending noradrenergic projections in the dorsolateral funiculus inhibit dorsal horn pain transmission, duloxetine and milnacipran are first-line for diabetic peripheral neuropathy and fibromyalgia (venlafaxine causes dose-dependent diastolic hypertension). Tricyclic antidepressants (TCAs)—tertiary amines (amitriptyline, imipramine, clomipramine) and secondary amines (nortriptyline, desipramine, which have significantly milder anticholinergic side effects)—also block SERT and NET but cause H₁ sedation/weight gain, M₁ anticholinergic toxicity, α₁ orthostatic hypotension, and lethal Class IA-like fast Na⁺ channel cardiotoxicity.

Atypical antidepressants solve specific clinical limitations of SSRIs. Bupropion is a norepinephrine-dopamine reuptake inhibitor (NDRI) with zero serotonergic activity; it causes zero sexual dysfunction and promotes weight loss, making it ideal as monotherapy or adjunctive therapy for SSRI-induced anorgasmia and smoking cessation, right alongside its strict contraindications in seizure disorders, anorexia nervosa, and bulimia nervosa (electrolyte shifts + bupropion dramatically lower the seizure threshold). Mirtazapine is an α₂-adrenergic autoreceptor/heteroreceptor antagonist (increasing synaptic NE and 5-HT release) that simultaneously blocks postsynaptic 5-HT₂, 5-HT₃, and H₁ receptors—producing potent sedation and increased appetite/weight gain Without sexual dysfunction (ideal for depressed elderly or cachectic patients with insomnia). Trazodone blocks 5-HT_2A, α₁, and H₁ receptors for insomnia but carries the classic board association with priapism ("trazo-bone" from α₁ blockade). Monoamine oxidase inhibitors (MAOIs: phenelzine, tranylcypromine, isocarboxazid, and selective MAO-B inhibitor selegiline) irreversibly inhibit monoamine breakdown and show superior efficacy in atypical depression (leaden paralysis, hypersomnia, hyperphagia, rejection sensitivity). However, inhibition of intestinal/hepatic MAO-A allows dietary tyramine (aged cheeses, cured meats, Chianti wine, fava beans, fermented soy) to enter the systemic circulation and displace vesicular norepinephrine via NET—precipitating a hypertensive crisis treated with IV phentolamine or nicardipine.

Understanding extrapyramidal symptoms (EPS) and endocrine side effects of antipsychotics requires mapping the four central dopaminergic tracts alongside positron emission tomography (PET) D₂ receptor occupancy thresholds. Therapeutic control of positive psychotic symptoms (auditory hallucinations, paranoid delusions) requires 60% to 75% D₂ receptor blockade in the mesolimbic pathway (ventral tegmental area [VTA] to nucleus accumbens). However, non-selective D₂ blockade in the mesocortical pathway (VTA to prefrontal cortex, which is already hypoactive in schizophrenia) worsens negative symptoms (abulia, alogia, flat affect) and cognitive deficits. When D₂ receptor occupancy exceeds 72% in the tuberoinfundibular pathway (arcuate nucleus of hypothalamus to anterior pituitary via the hypophyseal portal system), loss of tonic dopamine inhibition on lactotrophs causes hyperprolactinemia—which suppresses hypothalamic GnRH pulsatility and produces galactorrhea, secondary amenorrhea, gynecomastia, impotence, and accelerated osteopenia (most pronounced with risperidone, paliperidone, and haloperidol; uniquely spared or reversed by the D₂ partial agonist aripiprazole). Finally, when D₂ occupancy exceeds 78–80% in the nigrostriatal pathway (substantia nigra pars compacta to dorsal striatum), loss of inhibitory D₂ signaling disinhibits striatal cholinergic interneurons, triggering the chronological cascade of EPS.

Each extrapyramidal movement disorder follows a distinct onset timeline and requires targeted pharmacotherapy—never reflexively administering benztropine for every movement disorder. Within hours to 5 days of initiating or escalating a high-potency first-generation antipsychotic (haloperidol, fluphenazine) or antiemetic (metoclopramide, prochlorperazine), young male patients are especially prone to acute dystonia (torticollis, retrocollis, trismus, or upward conjugate oculogyric crisis), which responds within minutes to parenteral benztropine (M₁ antagonist) or diphenhydramine (H₁/M₁ antagonist). Within days to 8 weeks, patients may develop akathisia (intense subjective motor restlessness and inability to stop pacing or shifting weight), which does not reliably respond to anticholinergics and is treated first-line with propranolol (β-blocker) or lorazepam alongside antipsychotic dose reduction. Within weeks to months, drug-induced parkinsonism (symmetric bradykinesia, cogwheel rigidity, masked facies, shuffling gait) emerges and is treated with oral benztropine, trihexyphenidyl, or amantadine. Finally, after months to years of chronic D₂ blockade, compensatory upregulation and supersensitivity of postsynaptic striatal D₂ receptors produces tardive dyskinesia (TD)—involuntary choreoathetoid lip smacking, grimacing, and tongue protrusion. In TD, anticholinergics like benztropine worsen the dyskinesia and must be discontinued; definitive therapy is switching to clozapine or quetiapine and initiating selective reversible VMAT2 inhibitors (valbenazine or deutetrabenazine) to deplete presynaptic dopamine vesicles.

Second-generation (atypical) antipsychotics combine weaker, rapidly dissociating D₂ antagonism with potent 5-HT_2A receptor antagonism, reducing nigrostriatal EPS while improving negative symptoms. Clozapine is the single most effective antipsychotic—uniquely FDA-approved for treatment-resistant schizophrenia (failure of ≥2 adequate antipsychotic trials) and for reducing recurrent suicidal behavior in schizophrenia/schizoaffective disorder. However, clozapine carries five high-yield toxicities requiring vigilant monitoring: (1) Agranulocytosis (~1% risk, independent of dose, requiring mandatory baseline and weekly/biweekly absolute neutrophil count [ANC] monitoring; clozapine must be stopped immediately if ANC falls <1,000/mm³), (2) Myocarditis/cardiomyopathy (presenting with fever, chest pain, tachycardia, and troponin elevation in the first month), (3) Dose-dependent generalized seizures (3–5%), (4) Severe metabolic syndrome (shared with olanzapine: massive weight gain, insulin resistance, and hypertriglyceridemia mediated by H₁ and 5-HT_2C blockade), and (5) Nocturnal sialorrhea and severe anticholinergic constipation. Among Bipolar I mood stabilizers, lithium (which inhibits inositol monophosphatase and GSK-3β, has a narrow therapeutic window of 0.6–1.2 mEq/L, is cleared exclusively by the proximal tubule alongside Na⁺—so thiazides, NSAIDs, ACE inhibitors, and dehydration precipitate lithium toxicity—and causes nephrogenic DI, hypothyroidism, fine tremor, and maternal Ebstein anomaly), valproate (neural tube defects/spina bifida, hepatotoxicity, pancreatitis), carbamazepine (agranulocytosis, SIADH, SJS in HLA-B*1502), and lamotrigine (Stevens-Johnson syndrome requiring slow titration) require strict baseline and teratogenic counseling.

Differentiating Serotonin Syndrome (SS) from Neuroleptic Malignant Syndrome (NMS) in a febrile, diaphoretic, tachycardic, encephalopathic patient with labile blood pressure is one of the highest-yield emergency toxicology comparisons on USMLE and COMLEX. Both syndromes share autonomic instability, hyperthermia, diaphoresis, and altered mental status, so the definitive bedside discriminator is the neuromuscular and deep tendon reflex examination alongside onset kinetics and medication history. Serotonin Syndrome develops rapidly (within 2 to 24 hours) after combining two serotonergic agents (e.g., SSRI/SNRI + MAOI without a 2-week washout [5 weeks for fluoxetine], or SSRI + linezolid, tramadol, meperidine, dextromethorphan, triptans, MDMA, or St. John's Wort). Examination reveals neuromuscular hyperactivity: diffusely brisk hyperreflexia (3+/4+), inducible or spontaneous ankle and ocular clonus, myoclonus, tremor, and hyperactive bowel sounds/diarrhea; treatment is immediate cessation of serotonergic drugs, benzodiazepines, IV fluids, and the 5-HT_2A antagonist cyproheptadine. Conversely, Neuroleptic Malignant Syndrome evolves more gradually (over days to 2 weeks) following dopamine D₂ antagonist initiation/escalation (haloperidol, fluphenazine, risperidone, metoclopramide, or abrupt withdrawal of levodopa-carbidopa in Parkinson disease). Examination reveals "lead-pipe" generalized muscle rigidity, bradyreflexia/hyporeflexia, normal pupils, and massive rhabdomyolysis (CK >1,000–100,000 U/L); treatment is immediate discontinuation of the offending agent, aggressive cooling/hydration, dantrolene (RyR1 Ca²⁺ release blocker), and/or bromocriptine (D₂ agonist).

Medication-assisted treatment (MAT) for substance use disorders significantly reduces relapse and all-cause mortality. In Alcohol Use Disorder (AUD), three FDA-approved maintenance medications have distinct mechanisms and organ contraindications: (1) Naltrexone (an oral or monthly long-acting IM competitive μ-opioid receptor antagonist) blocks endogenous β-endorphin reinforcement in the mesolimbic reward pathway to reduce alcohol cravings and heavy drinking days (can be initiated while the patient is still drinking, but is contraindicated in acute hepatitis/cirrhosis and in patients taking any prescription or illicit opioids, as it precipitates immediate severe opioid withdrawal); (2) Acamprosate modulates hyperexcitable central NMDA glutamate and GABA_A receptors during post-detoxification abstinence to prevent relapse (cleared unchanged by the kidneys—making it the drug of choice in patients with alcoholic liver disease/cirrhosis, though contraindicated if CrCl <30 mL/min); and (3) Disulfiram irreversibly inhibits aldehyde dehydrogenase (ALDH2) to cause aversive acetaldehyde toxicity (flushing, throbbing headache, vomiting, hypotension) upon alcohol ingestion. In Opioid Use Disorder (OUD), long-term maintenance utilizes buprenorphine (a high-affinity partial μ-opioid agonist and κ-antagonist co-formulated sublingually with naloxone to deter IV injection; has a respiratory depression ceiling effect, but because its binding affinity exceeds full agonists, it must only be induced after the patient is already in mild-to-moderate opioid withdrawal [COWS >8–12] to avoid precipitated withdrawal) or methadone (a long-acting full μ-agonist and NMDA antagonist that prolongs the QTc interval).

Anxiolytic and sedative-hypnotic selection balances rapid symptom relief against dependence and fall risk. Benzodiazepines bind allosterically between the α and γ subunits of pentameric GABA_A Cl⁻ channels to increase the frequency of channel opening (requiring endogenous GABA; contrast with barbiturates, which increase the duration of Cl⁻ channel opening and directly open channels at high doses, causing lethal medullary respiratory depression without an antidote). For chronic Generalized Anxiety Disorder (GAD), SSRIs/SNRIs and buspirone—a non-sedating, non-addictive 5-HT_1A partial agonist that takes 2 to 4 weeks for clinical onset, does not interact with alcohol, and does not treat acute panic attacks or alcohol withdrawal—are preferred over chronic benzodiazepines. For insomnia, "Z-drugs" (zolpidem, zaleplon, eszopiclone) selectively bind the BZ₁ (α₁) subunit of GABA_A (reversed by flumazenil), ramelteon activates suprachiasmatic nucleus MT₁/MT₂ melatonin receptors (safe in elderly/substance use history; zero abuse potential), and suvorexant/lemborexant block wakefulness-promoting orexin OX₁/OX₂ receptors (contraindicated in narcolepsy). In ADHD, first-line stimulants (methylphenidate [blocks NET/DAT reuptake] and amphetamines [blocks reuptake AND displaces vesicular monoamines via VMAT2 reversal]) improve prefrontal executive attention but suppress appetite/linear growth and exacerbate motor tics; non-stimulant alternatives for patients with Tourette syndrome, substance use risk, or insomnia include atomoxetine (selective NET inhibitor) and central α_2A agonists (guanfacine, clonidine).

Selective Serotonin Reuptake Inhibitors (SSRIs: fluoxetine, sertraline, paroxetine, citalopram, escitalopram, fluvoxamine) are first-line for major depressive disorder, generalized anxiety, panic disorder, OCD, PTSD, bulimia nervosa, and premenstrual dysphoric disorder. Key subclass nuances: Paroxetine has the shortest half-life, strongest CYP2D6 inhibition (blocks tamoxifen activation to endoxifen!), most anticholinergic weight gain, and severe SSRI discontinuation syndrome (dizziness, paresthesias/"brain zaps", dysphoria upon abrupt cessation); Fluoxetine has the longest half-life (norfluoxetine t_1/2 7–15 days; safe to taper and approved in pediatric depression); Citalopram causes dose-dependent QT prolongation (>40 mg/day); and Sertraline carries the lowest cardiac risk post-myocardial infarction. Classic SSRI side effects include anorgasmia/decreased libido (5-HT₂ stimulation in spinal cord; treat by adding bupropion or switching to mirtazapine/vilazodone), GI distress, SIADH (hyponatremia in elderly), and impaired platelet aggregation (depletion of platelet dense-granule serotonin increases GI bleeding risk when combined with NSAIDs or anticoagulants).

Serotonin-Norepinephrine Reuptake Inhibitors (SNRIs: venlafaxine, desvenlafaxine, duloxetine, levomilnacipran, milnacipran) inhibit both SERT and NET, making duloxetine and milnacipran first-line for diabetic peripheral neuropathy, fibromyalgia, and chronic musculoskeletal pain (descending noradrenergic pain inhibition in the dorsal horn). Because they increase sympathetic tone, SNRIs (especially venlafaxine) cause dose-dependent diastolic hypertension.

Tricyclic Antidepressants (TCAs) include tertiary amines (amitriptyline, imipramine, clomipramine, doxepin—more serotonergic and heavily anticholinergic/sedating) and secondary amines (nortriptyline, desipramine—more noradrenergic with fewer anticholinergic effects, preferred in older adults if a TCA is required). TCAs are utilized for migraine prophylaxis, neuropathic pain, OCD (clomipramine), and nocturnal enuresis (imipramine), but cause the lethal "3 Cs" in overdose: Coma, Convulsions (GABA_A blockade), and Cardiotoxicity (fast Na⁺ channel QRS widening — treat with IV NaHCO₃) alongside alpha₁ orthostasis, H₁ weight gain/sedation, and M₁ antimuscarinic toxicity.

Monoamine Oxidase Inhibitors (MAOIs: phenelzine, tranylcypromine, isocarboxazid [non-selective MAO-A/B]; selegiline, rasagiline [selective MAO-B at low doses]) are reserved for atypical depression (characterized by mood reactivity, hypersomnia, hyperphagia, and "leaden paralysis" of limbs). Because intestinal and hepatic MAO-A normally degrades dietary tyramine (found in aged cheeses, cured meats, red wine, fava beans, fermented soy/kimchi, draft beer), patients on MAOIs who ingest tyramine absorb massive amounts into the portal circulation; tyramine enters sympathetic nerve terminals via NET and displaces vesicular norepinephrine, precipitating a life-threatening hypertensive crisis (treat with phentolamine or nicardipine).

Each atypical antidepressant possesses a signature board identity: Bupropion (norepinephrine-dopamine reuptake inhibitor, NDRI) causes ZERO sexual dysfunction and promotes weight loss, making it ideal for depressed patients with hypersomnia/fatigue, SSRI-induced anorgasmia, or smoking cessation (contraindicated in seizures and bulimia/anorexia). Mirtazapine blocks presynaptic alpha₂-autoreceptors and heteroreceptors (enhancing both NE and 5-HT release) while potently antagonizing postsynaptic 5-HT₂, 5-HT₃, and H₁ receptors; it causes marked sedation and increased appetite/weight gain without sexual dysfunction, making it ideal for elderly, cachectic, or cancer patients with depression, insomnia, and anorexia/nausea. Trazodone blocks 5-HT_2A, alpha₁, and H₁ receptors (used at low doses for insomnia; causes orthostatic hypotension and priapism ["trazo-bone"]). Vilazodone (SSRI + 5-HT_1A partial agonist) and Vortioxetine (SSRI + 5-HT₃ antagonist / 5-HT_1A agonist) minimize sexual side effects. Finally, intranasal esketamine (NMDA glutamate receptor antagonist) and brexanolone / zuranolone (allopregnanolone neurosteroid positive allosteric modulators of synaptic and extrasynaptic GABA_A receptors) provide rapid relief in treatment-resistant depression and postpartum depression, respectively.

First-Generation (Typical) Antipsychotics are stratified by D₂ affinity: High-potency agents (haloperidol, fluphenazine, pimozide, trifluoperazine) carry the highest risk of extrapyramidal symptoms, hyperprolactinemia, QT prolongation/Torsades (IV haloperidol), and Neuroleptic Malignant Syndrome (NMS). Conversely, Low-potency agents (chlorpromazine, thioridazine) have lower EPS risk but potent H₁, M₁, and alpha₁ blockade (sedation, anticholinergic delirium, orthostatic hypotension) plus unique ocular toxicities: chlorpromazine causes harmless corneal and lenticular deposits, whereas thioridazine causes irreversible blinding retinal pigmentary deposits (retinitis pigmentosa-like retinopathy) and severe cardiotoxic QT prolongation.

Second-Generation (Atypical) Antipsychotics antagonize both 5-HT_2A and D₂ receptors (or act as a D₂ partial agonist: aripiprazole, brexpiprazole, cariprazine), reducing nigrostriatal EPS and improving negative symptoms. Board discriminators within atypicals include: Clozapine and Olanzapine cause the most severe metabolic syndrome (weight gain, insulin-resistant hyperglycemia/DKA, dyslipidemia); Risperidone and Paliperidone (9-hydroxyrisperidone) behave most like typicals at high doses, causing the highest rate of hyperprolactinemia and EPS among atypicals; Quetiapine and Pimavanserin (selective 5-HT_2A inverse agonist) have minimal D₂ nigrostriatal occupancy and are preferred for Parkinson disease psychosis and Lewy body dementia; Ziprasidone causes minimal weight gain but marked QTc prolongation (and must be taken with food); and Aripiprazole and Lurasidone offer the most favorable metabolic profiles.

Extrapyramidal symptoms evolve along a strict board timeline: (1) Acute Dystonia (hours to days): sudden sustained muscle spasm (torticollis, trismus, laryngospasm, oculogyric crisis)—treat immediately with parenteral antimuscarinics (benztropine) or antihistamines (diphenhydramine); (2) Akathisia (days to weeks): subjective inner motor restlessness and inability to sit still (often misdiagnosed as worsening psychotic agitation!)—treat FIRST with propranolol (lipophilic beta-blocker), or benzodiazepines/benztropine; (3) Drug-Induced Parkinsonism (weeks to months): bradykinesia, cogwheel rigidity, shuffling gait, pill-rolling resting tremor—treat with benztropine or amantadine; and (4) Tardive Dyskinesia (months to years): chronic nigrostriatal D₂ blockade induces compensatory postsynaptic D₂ receptor supersensitivity and upregulation, producing choreoathetoid orofacial dyskinesias (lip smacking, tongue protrusion, grimacing). Crucially, anticholinergics (benztropine) WORSEN tardive dyskinesia! Definitive management of tardive dyskinesia requires tapering/switching to clozapine or quetiapine and administering a reversible Vesicular Monoamine Transporter-2 (VMAT2) inhibitor: valbenazine or deutetrabenazine (or tetrabenazine), which depletes presynaptic striatal dopamine vesicles.

Benzodiazepines bind allosterically between the alpha and gamma subunits of the pentameric GABA_A ligand-gated Cl⁻ channel, increasing the FREQUENCY of channel opening in the presence of endogenous GABA (whereas barbiturates—phenobarbital, pentobarbital, thiopental—increase the DURATION of Cl⁻ channel opening and directly activate the channel at high doses, explaining why barbiturates carry a far higher risk of fatal respiratory depression and lack a specific reversal antidote). High-yield pharmacokinetic rule: Oxazepam, Temazepam, and Lorazepam ("Outside The Liver") undergo direct Phase II glucuronidation without Phase I CYP450 oxidation or active metabolites, making them the benzodiazepines of choice in decompensated cirrhosis and elderly patients (specifically lorazepam or oxazepam for alcohol withdrawal delirium tremens in alcoholic liver disease).

Non-benzodiazepine "Z-drugs" (zolpidem, zaleplon, eszopiclone) selectively bind the BZ₁ (α₁-subunit) receptor subtype on the GABA_A complex to produce hypnosis without anxiolytic, anticonvulsant, or muscle-relaxant properties (reversed by flumazenil; cause complex sleep-related behaviors/sleepwalking and delirium in older adults). Safe non-GABAergic insomnia agents with zero dependence risk include ramelteon (selective MT₁/MT₂ melatonin receptor agonist in the suprachiasmatic nucleus; safe in geriatric patients) and suvorexant / lemborexant (dual orexin OX₁R/OX₂R antagonists that suppress wakefulness drive; contraindicated in narcolepsy). For chronic generalized anxiety disorder, buspirone acts as a 5-HT_1A partial agonist that takes 1–2 weeks for clinical onset, does not interact with alcohol, and causes zero sedation, cognitive impairment, or withdrawal (ineffective for acute panic attacks or alcohol withdrawal).

First-line pharmacotherapy for ADHD in children (≥6 years) and adults consists of CNS stimulants that elevate prefrontal cortex norepinephrine and dopamine: methylphenidate / dexmethylphenidate (blocks NET and DAT reuptake) and amphetamine salts / lisdexamfetamine (blocks NET/DAT reuptake AND enters presynaptic vesicles via VMAT2 to actively displace catecholamines into the synapse). Stimulants reduce appetite, suppress growth velocity, cause insomnia and tachycardia, and may unmask motor tics. Non-stimulant alternatives (preferred in comorbid tic disorders/Tourette syndrome, severe anxiety, or substance abuse risk) include atomoxetine (selective NET inhibitor; carries an adolescents suicidal ideation warning and rare hepatotoxicity) and central alpha_2A-adrenergic agonists (guanfacine, clonidine), which strengthen prefrontal cortical network connectivity and suppress tics.

For Alcohol Use Disorder (AUD) relapse prevention, three FDA-approved agents exist: (1) Naltrexone (mu-opioid receptor antagonist that extinguishes mesolimbic dopamine reward/craving; patients can initiate while still drinking, though it is contraindicated in acute hepatitis/decompensated cirrhosis or current opioid use), (2) Acamprosate (NMDA glutamate receptor modulator and GABA_A enhancer that reduces protracted post-withdrawal dysphoria; cleared 100% renally—safe in severe liver disease, contraindicated if CrCl <30 mL/min), and (3) Disulfiram (irreversibly inhibits aldehyde dehydrogenase, accumulating toxic acetaldehyde upon alcohol ingestion; requires high patient motivation). For Opioid Use Disorder (OUD), methadone (full mu-agonist; prolongs QTc) and buprenorphine + naloxone (Suboxone) (high-affinity partial mu-agonist with a respiratory-depression ceiling effect; naloxone is poorly absorbed sublingually and only precipitates withdrawal if crushed and injected IV; note that initiating buprenorphine too soon while full agonists occupy receptors precipitates acute withdrawal!). For Tobacco Use Disorder, varenicline (α₄β₂ nicotinic acetylcholine receptor partial agonist) is the most effective single pharmacotherapy, alongside bupropion and dual nicotine replacement therapy (transdermal patch + short-acting gum/lozenge).