Section 4.11: Clinical Pharmacology: Neurological & Psychiatric Disorders
Key Takeaways
- Antiseizure medications require close therapeutic drug monitoring and pharmacogenomic screening, such as HLA-B*1502 testing prior to carbamazepine initiation in at-risk populations.
- Parkinson's disease management utilizes carbidopa/levodopa as the most effective agent, but dopamine agonists are associated with impulse control disorders and sudden sleep attacks.
- Second-generation antipsychotics carry class-wide metabolic risks (weight gain, dyslipidemia, hyperglycemia), with clozapine and olanzapine presenting the highest risk.
- Clozapine requires strict absolute neutrophil count (ANC) monitoring through a REMS registry due to the risk of life-threatening agranulocytosis.
Last updated: July 2026
Clinical Pharmacology: Neurological & Psychiatric Disorders
Neurological and psychiatric pharmacotherapy involves drugs with narrow therapeutic indexes, severe side effect profiles, and significant drug-drug interactions. Pharmacists must be competent in selecting, dosing, and monitoring these agents.
1. Epilepsy & Antiseizure Medications (ASMs)
ASMs are divided into broad-spectrum (effective for both focal and generalized seizures) and narrow-spectrum (effective primarily for focal seizures; may worsen absence or myoclonic seizures).
ASM Class Comparison & Safety Profiles
| Drug | Spectrum | Key Adverse Effects | Therapeutic Range & Monitoring |
|---|---|---|---|
| Carbamazepine | Narrow | Hyponatremia (SIADH), aplastic anemia, rash. HLA-B*1502 screening required in patients of Asian/Middle Eastern descent due to risk of Stevens-Johnson Syndrome (SJS/TEN). | 4–12 mcg/mL. Induces its own metabolism (autoinduction) over 3–4 weeks. |
| Phenytoin | Narrow | Acute: Nystagmus, ataxia, diplopia. Chronic: Gingival hyperplasia, hirsutism, peripheral neuropathy, osteomalacia. | 10–20 mcg/mL (total) or 1–2.5 mcg/mL (free). Exhibits Michaelis-Menten kinetics (non-linear: small dose increases can lead to disproportionate serum level increases). |
| Valproic Acid | Broad | Hepatotoxicity (fatal liver failure in children <2), pancreatitis, thrombocytopenia, weight gain, alopecia, hyperammonemia. Highly teratogenic (neural tube defects). | 50–100 mcg/mL. Strong enzyme inhibitor. |
| Lamotrigine | Broad | Serious rash (SJS/TEN). Requires strict, slow dose titration. | Standard titration must be halved if co-administered with valproic acid (inhibitor) or doubled if with carbamazepine (inducer). |
| Topiramate | Broad | Nephrolithiasis (kidney stones), metabolic acidosis (carbonic anhydrase inhibition), oligohidrosis (reduced sweating, hyperthermia in children), cognitive slowing ("dope-a-max"), weight loss. | Ensure adequate hydration to prevent kidney stones. |
| Levetiracetam | Broad | Somnolence, fatigue, neuropsychiatric symptoms (irritability, aggression, "Keppra rage"). | Renal adjustment required. Minimal drug-drug interactions. |
2. Parkinson's Disease (PD)
PD is characterized by a loss of dopaminergic neurons in the substantia nigra. Pharmacotherapy aims to restore dopamine levels or block cholinergic activity.
- Carbidopa/Levodopa: Levodopa is the precursor to dopamine; carbidopa inhibits peripheral DOPA decarboxylase, allowing levodopa to cross the blood-brain barrier. It is the most effective drug for motor symptoms.
- Complications: Long-term use leads to motor fluctuations ("wearing-off" effect) and dyskinesias. "Wearing-off" can be managed by adding a COMT inhibitor (Entacapone), an MAO-B inhibitor (Rasagiline), or increasing the frequency of levodopa doses.
- Counseling: Avoid high-protein meals at the time of dosing, as amino acids compete with levodopa for absorption in the gut and crossing the blood-brain barrier.
- Dopamine Agonists (Pramipexole, Ropinirole, Rotigotine): Directly stimulate dopamine receptors. Associated with impulse control disorders (pathological gambling, compulsive shopping, hypersexuality) and sudden sleep attacks.
- Anticholinergics (Benztropine, Trihexyphenidyl): Used primarily in younger patients (< 65 years) where tremor is the predominant symptom. Avoid in the elderly due to severe cognitive impairment, dry mouth, urinary retention, and constipation.
- Amantadine: Promotes dopamine release and blocks NMDA receptors. Used to treat levodopa-induced dyskinesias. Side effect includes livedo reticularis.
3. Alzheimer's Disease (AD)
AD involves cholinergic deficit and glutamatergic excitotoxicity.
- Cholinesterase Inhibitors (Donepezil, Rivastigmine, Galantamine): First-line for mild-to-moderate AD. Inhibit acetylcholinesterase, increasing acetylcholine levels. Side effects: GI distress (nausea, diarrhea), bradycardia, heart block, and vivid dreams (donepezil). Take donepezil at bedtime to minimize daytime GI symptoms.
- NMDA Receptor Antagonist (Memantine): Blocks pathological glutamate stimulation. Used for moderate-to-severe AD, often added to donepezil. Side effects: dizziness, headache, confusion.
4. Psychiatric Disorders
Major Depressive Disorder (MDD) & Anxiety
- SSRIs (Escitalopram, Sertraline, Fluoxetine, Paroxetine, Citalopram): First-line for both depression and anxiety. Side effects: sexual dysfunction, weight gain, insomnia, and QT prolongation (specifically Citalopram, maximum dose 40 mg daily, or 20 mg in patients > 60 years). Avoid abrupt withdrawal (discontinuation syndrome; least likely with fluoxetine due to long half-life).
- SNRIs (Venlafaxine, Duloxetine): Inhibit both serotonin and norepinephrine reuptake. Venlafaxine can cause dose-dependent increases in blood pressure. Duloxetine is also indicated for neuropathic pain but must be avoided in hepatic impairment.
- Bupropion: Inhibits dopamine and norepinephrine reuptake. Used for depression and smoking cessation. Side effects: dry mouth, insomnia, tremors. Contraindicated in patients with a history of seizures, bulimia, or anorexia nervosa (lowers seizure threshold).
- Mirtazapine: Alpha-2 antagonist, increases serotonin and norepinephrine release. Highly sedating at low doses (7.5–15 mg) and causes weight gain (increased appetite), making it useful for depressed patients with insomnia and cachexia.
- Tricyclic Antidepressants (TCAs - Amitriptyline, Nortriptyline) & MAOIs (Phenelzine): Second/third-line due to toxicity. TCAs have anticholinergic side effects and cause fatal arrhythmias in overdose (cardiotoxicity via sodium channel blockade). MAOIs require a tyramine-restricted diet to prevent hypertensive crisis.
- Benzodiazepines (BZDs - Lorazepam, Alprazolam, Diazepam): Used for acute anxiety. Highly addictive, cause tolerance, dependence, and cognitive impairment. Avoid in the elderly (Beers Criteria) due to risk of falls and delirium.
Schizophrenia & Antipsychotics
| Antipsychotic Class | Key Features | Metabolic Risk | Extrapyramidal Symptoms (EPS) | Hyperprolactinemia |
|---|---|---|---|---|
| First-Generation (FGAs) (e.g., Haloperidol, Fluphenazine) | Dopamine D2 antagonists; highly effective for positive symptoms | Low | High (dystonia, akathisia, parkinsonism, tardive dyskinesia) | High (gynecomastia, galactorrhea, sexual dysfunction) |
| Second-Generation (SGAs) (e.g., Olanzapine, Clozapine, Risperidone) | Serotonin-dopamine antagonists; lower EPS risk; higher metabolic risk | High (especially Clozapine, Olanzapine; lowest with Aripiprazole, Ziprasidone) | Low to Moderate (highest with Risperidone/Paliperidone) | Low to Moderate (highest with Risperidone/Paliperidone) |
- Clozapine (SGA): Most effective antipsychotic, reserved for treatment-resistant schizophrenia. Induces virtually no EPS. However, it carries a black box warning for agranulocytosis. Under SFDA and international guidelines, clozapine requires strict registration and monitoring of Absolute Neutrophil Count (ANC):
- Monitoring Schedule: Weekly for the first 6 months, biweekly for the next 6 months, then monthly. Treatment must be interrupted if ANC falls below 1500 cells/mcL (or < 1000 cells/mcL for patients with documented benign ethnic neutropenia).
- Other Warnings: Myocarditis, severe constipation, and dose-related seizures.
- Neuroleptic Malignant Syndrome (NMS): Rare, life-threatening reaction to antipsychotics. Symptoms: hyperthermia, "lead-pipe" muscle rigidity, altered mental status, and autonomic instability. Treatment: discontinue offending agent, initiate supportive care, and administer dantrolene or bromocriptine.
Test Your Knowledge
A 28-year-old female patient is to be initiated on carbamazepine for focal seizures. She is of Middle Eastern descent. Which of the following genetic screenings is highly recommended prior to starting carbamazepine?
A
B
C
D
Test Your Knowledge
A patient with schizophrenia has failed adequate trials of risperidone and aripiprazole. The psychiatrist wants to initiate clozapine. Which of the following baseline laboratory values is mandatory to verify before dispensing the first dose of clozapine?
A
B
C
D
Test Your Knowledge
A 34-year-old patient with depression is prescribed bupropion. Which of the following clinical conditions in the patient's medical history represents an absolute contraindication to the use of bupropion?
A
B
C
D