TOPAMAX

Clinical safety rating: caution

Comprehensive clinical and safety monograph for TOPAMAX (TOPAMAX).

How it works

Antiepileptic; modulates voltage-gated sodium channels, enhances GABA-A activity, antagonizes AMPA/kainate glutamate receptors, weakly inhibits carbonic anhydrase.

What the body does with it

Metabolism	Minimally metabolized (~20%) via hydroxylation, hydrolysis, and glucuronidation; not extensively metabolized by CYP450; ~70% excreted unchanged in urine.
Excretion	Renal: ~70% (unchanged drug); remainder as metabolites. Biliary/fecal: minimal (<5%).
Half-life	Terminal elimination half-life is 21 hours (range 18-23 hours). Linear pharmacokinetics. Half-life is prolonged in renal impairment (CrCl <70 mL/min: ~35 hours).
Protein binding	15-41% bound; primarily to albumin. Binding is low and not clinically significant.
Volume of Distribution	0.6-0.8 L/kg. Indicates distribution into total body water, suggesting minimal tissue binding.
Bioavailability	Oral: ~80% (range 65-95%). Bioavailability is not affected by food. Tablets and sprinkle capsules are bioequivalent.
Onset of Action	Oral: Peak plasma concentrations achieved in 2-4 hours. Clinical onset of anticonvulsant effect typically within 1-2 weeks of dosing.
Duration of Action	Duration of anticonvulsant effect: ~24 hours with twice-daily dosing due to half-life. For migraine prophylaxis, therapeutic effect may take 2-4 weeks to manifest.

Classification & Brands

Dosing & administration

Initial dose 25 mg orally twice daily; titrate by 25-50 mg weekly to effective dose; usual maintenance dose 200-400 mg/day divided twice daily; maximum 1600 mg/day.

Dosage form	CAPSULE
Renal impairment	CrCl ≥70 mL/min: no adjustment. CrCl 30-69 mL/min: reduce dose by 50%. CrCl <30 mL/min: reduce dose by 75%. Hemodialysis: administer 50% supplemental dose after dialysis.
Liver impairment	Child-Pugh Class A: no adjustment. Child-Pugh Class B: reduce dose by 50%. Child-Pugh Class C: reduce dose by 75%.
Pediatric use	For epilepsy (age 2-16): initial dose 1-3 mg/kg/day divided twice daily; titrate by 1-3 mg/kg/day every 1-2 weeks; maintenance typically 5-9 mg/kg/day divided twice daily. For migraine prophylaxis (age 12-17): initial 25 mg/day at bedtime; titrate to 50-100 mg/day divided twice daily.
Geriatric use	Initiate at lower dose (25 mg daily); titrate slowly due to increased sensitivity and higher risk of adverse effects; monitor renal function and adjust based on CrCl; consider reduced target doses.

Use during pregnancy

1st trimester	Consult provider
2nd trimester	Consult provider
3rd trimester	Consult provider

Clinical note

Comprehensive clinical and safety monograph for TOPAMAX (TOPAMAX).

Breastfeeding	Topiramate is excreted in breast milk with an M/P ratio of approximately 0.9. Relative infant dose is 10-20% of maternal weight-adjusted dose. Monitor for infant sedation, poor feeding, and diarrhea. Use with caution, especially in neonates.
Teratogenic Risk	Pregnancy Category D. First trimester exposure increases risk of oral clefts (6- to 16-fold). Second/third trimester exposure associated with low birth weight, microcephaly, and developmental delay. Risk of major congenital malformations 3-5% vs 1-2% baseline.

Warnings & precautions

■ FDA Black Box Warning

Acute myopia and secondary angle-closure glaucoma; oligohidrosis and hyperthermia; metabolic acidosis; fetal risk (pregnancy category D).

Side Effect Profile

Serious Effects

Absolute Contraindications

Hypersensitivity to topiramate or any component of the formulation; acute angle-closure glaucoma; concurrent use with metformin (contraindicated due to metabolic acidosis risk) per FDA labeling.

Clinical Precautions

Precautions

Monitor for oligohidrosis and hyperthermia; monitor for metabolic acidosis (measure serum bicarbonate); avoid abrupt discontinuation (seizure frequency increase); nephrolithiasis risk; acute myopia and secondary angle-closure glaucoma; cognitive/neuropsychiatric adverse events; teratogenicity (cleft lip/palate); decreased efficacy of oral contraceptives; hyperammonemia with encephalopathy with valproic acid use; ketoacidosis risk.

Clinical Tips & Counseling

Drug interactions

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TOPAMAX

Clinical safety rating: caution

Comprehensive clinical and safety monograph for TOPAMAX (TOPAMAX).

How it works

Antiepileptic; modulates voltage-gated sodium channels, enhances GABA-A activity, antagonizes AMPA/kainate glutamate receptors, weakly inhibits carbonic anhydrase.

What the body does with it

Metabolism	Minimally metabolized (~20%) via hydroxylation, hydrolysis, and glucuronidation; not extensively metabolized by CYP450; ~70% excreted unchanged in urine.
Excretion	Renal: ~70% (unchanged drug); remainder as metabolites. Biliary/fecal: minimal (<5%).
Half-life	Terminal elimination half-life is 21 hours (range 18-23 hours). Linear pharmacokinetics. Half-life is prolonged in renal impairment (CrCl <70 mL/min: ~35 hours).
Protein binding	15-41% bound; primarily to albumin. Binding is low and not clinically significant.
Volume of Distribution	0.6-0.8 L/kg. Indicates distribution into total body water, suggesting minimal tissue binding.
Bioavailability	Oral: ~80% (range 65-95%). Bioavailability is not affected by food. Tablets and sprinkle capsules are bioequivalent.
Onset of Action	Oral: Peak plasma concentrations achieved in 2-4 hours. Clinical onset of anticonvulsant effect typically within 1-2 weeks of dosing.
Duration of Action	Duration of anticonvulsant effect: ~24 hours with twice-daily dosing due to half-life. For migraine prophylaxis, therapeutic effect may take 2-4 weeks to manifest.

Classification & Brands

Dosing & administration

Initial dose 25 mg orally twice daily; titrate by 25-50 mg weekly to effective dose; usual maintenance dose 200-400 mg/day divided twice daily; maximum 1600 mg/day.

Dosage form	CAPSULE
Renal impairment	CrCl ≥70 mL/min: no adjustment. CrCl 30-69 mL/min: reduce dose by 50%. CrCl <30 mL/min: reduce dose by 75%. Hemodialysis: administer 50% supplemental dose after dialysis.
Liver impairment	Child-Pugh Class A: no adjustment. Child-Pugh Class B: reduce dose by 50%. Child-Pugh Class C: reduce dose by 75%.
Pediatric use	For epilepsy (age 2-16): initial dose 1-3 mg/kg/day divided twice daily; titrate by 1-3 mg/kg/day every 1-2 weeks; maintenance typically 5-9 mg/kg/day divided twice daily. For migraine prophylaxis (age 12-17): initial 25 mg/day at bedtime; titrate to 50-100 mg/day divided twice daily.
Geriatric use	Initiate at lower dose (25 mg daily); titrate slowly due to increased sensitivity and higher risk of adverse effects; monitor renal function and adjust based on CrCl; consider reduced target doses.

Use during pregnancy

1st trimester	Consult provider
2nd trimester	Consult provider
3rd trimester	Consult provider

Clinical note

Comprehensive clinical and safety monograph for TOPAMAX (TOPAMAX).

Breastfeeding	Topiramate is excreted in breast milk with an M/P ratio of approximately 0.9. Relative infant dose is 10-20% of maternal weight-adjusted dose. Monitor for infant sedation, poor feeding, and diarrhea. Use with caution, especially in neonates.
Teratogenic Risk	Pregnancy Category D. First trimester exposure increases risk of oral clefts (6- to 16-fold). Second/third trimester exposure associated with low birth weight, microcephaly, and developmental delay. Risk of major congenital malformations 3-5% vs 1-2% baseline.

Warnings & precautions

■ FDA Black Box Warning

Acute myopia and secondary angle-closure glaucoma; oligohidrosis and hyperthermia; metabolic acidosis; fetal risk (pregnancy category D).

Side Effect Profile

Serious Effects

Absolute Contraindications

Hypersensitivity to topiramate or any component of the formulation; acute angle-closure glaucoma; concurrent use with metformin (contraindicated due to metabolic acidosis risk) per FDA labeling.