Topiramate

Indications

• FDA-approved as monotherapy for partial-onset and primary generalized tonic-clonic seizures in patients aged 2 years and older.
• FDA-approved as adjunctive therapy for partial-onset seizures, generalized tonic-clonic seizures, and seizures of Lennox-Gastaut syndrome.
• FDA-approved for migraine prophylaxis in patients aged 12 and older, and in combination with phentermine for chronic weight management.
• Off-label but well supported for alcohol use disorder, where it reduces heavy drinking days at doses of 200 to 300 mg per day.
• Off-label use for binge eating disorder, antipsychotic-induced weight gain, and cocaine use disorder, with modest supporting trial data.
• Off-label use in bipolar disorder is not supported; controlled monotherapy trials in acute mania were negative.

Mechanism of action

• Blocks voltage-gated sodium channels and reduces sustained repetitive firing, the shared mechanism of most broad-spectrum anticonvulsants.
• Antagonizes AMPA and kainate glutamate receptors, dampening excitatory transmission implicated in craving and migraine generation.
• Enhances GABA-A receptor activity at a site distinct from the benzodiazepine site, adding inhibitory tone without benzodiazepine tolerance.
• Weakly inhibits carbonic anhydrase isoenzymes, producing metabolic acidosis, paresthesias, and calcium phosphate kidney stones.
• Appetite suppression and dose-dependent weight loss follow from combined glutamatergic, GABAergic, and taste-related effects.

Pharmacokinetics

• Absorption is rapid and nearly complete, is not affected by food, and gives peak plasma levels within about 2 hours.
• The elimination half-life is roughly 21 hours, supporting twice-daily immediate-release or once-daily extended-release dosing.
• Approximately 70 percent of a dose is excreted unchanged in urine, so renal function rather than hepatic metabolism drives clearance.
• Protein binding is low at 15 to 41 percent and metabolism is minor, which limits the number of pharmacokinetic interactions.
• It weakly induces CYP3A4 at doses above 200 mg per day and weakly inhibits CYP2C19, effects that matter mainly for contraceptives.

Dosing

• For migraine prophylaxis start 25 mg at bedtime and increase by 25 mg weekly to a usual target of 100 mg/day in two divided doses.
• For epilepsy monotherapy titrate over 6 weeks toward 400 mg/day divided twice daily, which is the labeled adult target dose.
• For alcohol use disorder, off-label practice titrates by 25 to 50 mg weekly to 200-300 mg/day, balancing efficacy against cognitive effects.
• Slow titration is the key to tolerability, since paresthesias and word-finding problems track with the speed of dose escalation.
• Halve the dose when creatinine clearance falls below 70 mL/min, and give a supplemental dose on hemodialysis days.
• Taper over at least 2 to 3 weeks when stopping, since abrupt withdrawal can precipitate seizures even in nonepileptic patients.

Adverse effects

• Paresthesias affect up to half of patients at higher doses and are the most common reason for early discontinuation.
• Cognitive effects with word-finding difficulty, slowed processing, and memory complaints are dose-related and often dose-limiting.
• Dose-dependent weight loss averages 2 to 5 kilograms and is sometimes the reason the drug is chosen in psychiatric practice.
• Taste alteration, especially of carbonated beverages, plus fatigue, dizziness, and somnolence are common early complaints.
• Metabolic acidosis from carbonic anhydrase inhibition occurs in a substantial minority, with kidney stones in about 1.5 percent.
• Acute angle-closure glaucoma and oligohidrosis with hyperthermia are rare but urgent, the former usually in the first month.

Monitoring

• Check serum bicarbonate at baseline, after titration, and periodically thereafter, and evaluate any unexplained hyperventilation or fatigue.
• Advise immediate evaluation for acute eye pain, blurred vision, or red eye, which may indicate secondary angle-closure glaucoma.
• Monitor weight and, in patients where weight loss is undesirable, track intake and consider a different agent if loss is excessive.
• Assess renal function at baseline, encourage generous fluid intake, and ask about flank pain or hematuria suggesting stones.
• In children and in hot climates, monitor for decreased sweating and elevated body temperature during exercise or heat exposure.

Interactions

• Doses above 200 mg per day can lower ethinyl estradiol exposure enough to reduce oral contraceptive reliability, so counsel on backup methods.
• Carbamazepine and phenytoin induce topiramate clearance and can lower its concentrations by roughly 40 to 50 percent.
• Combined with valproate, topiramate can precipitate hyperammonemia and encephalopathy even when both levels appear therapeutic.
• Other carbonic anhydrase inhibitors such as zonisamide and acetazolamide compound the risk of acidosis and nephrolithiasis.
• Alcohol adds sedation and cognitive impairment, and the extended-release Trokendi formulation should not be used within 6 hours of alcohol.

Special populations

• First-trimester exposure raises the risk of cleft lip and palate roughly threefold and increases the rate of small-for-gestational-age infants.
• Discuss contraception and pregnancy planning before starting in anyone who can become pregnant, and use the lowest effective dose.
• Topiramate passes into breast milk in meaningful amounts; monitor the infant for sedation, poor feeding, and diarrhea if nursing.
• Children clear topiramate faster than adults and are more susceptible to oligohidrosis, hyperthermia, and metabolic acidosis.
• In older adults and any patient with reduced creatinine clearance, halve the dose and titrate more slowly to limit cognitive effects.

Clinical pearls

• Titrate 25 mg per week; almost all the paresthesia and cognitive complaints come from moving faster.
• Best psychiatric evidence is for alcohol use disorder and binge eating, not for bipolar disorder.
• New eye pain or blurred vision in month one means urgent evaluation for angle-closure glaucoma.