topiramate
Brand: Topamax, Trokendi XR, Qudexy XR
Prototype: valproate
Drug Class: antiepileptic drug
Drug Family: antiepileptic
Subclass: broad-spectrum AED with multiple mechanisms
Organ Systems: cns
Mechanism of Action
Multiple complementary mechanisms: (1) blocks voltage-gated Na+ channels; (2) enhances GABA-A receptor activity; (3) antagonizes AMPA/kainate glutamate receptors; (4) inhibits carbonic anhydrase, which contributes to both anti-seizure effects and weight loss/metabolic acidosis adverse effects.
voltage-gated Na+ channelsGABA-A receptors (enhancement)AMPA/kainate glutamate receptors (blockade)carbonic anhydrase isoforms II and IV
Indications
- partial and generalized seizures
- Lennox-Gastaut syndrome
- migraine prophylaxis
- obesity (Qsymia — combination with phentermine)
Contraindications
- renal calculi (carbonic anhydrase inhibition increases kidney stone risk)
- metabolic acidosis (non-anion gap) in setting requiring alkali reserves
- pregnancy for migraine prevention (category D — oral cleft risk)
Adverse Effects
Common
- cognitive impairment (word-finding difficulty — called 'Dopamax')
- somnolence
- dizziness
- weight loss
- paresthesias
- fatigue
Serious
- kidney stones (nephrolithiasis — carbonic anhydrase inhibition)
- metabolic acidosis (hyperchloremic non-AG)
- acute myopia with angle-closure glaucoma (rare; medical emergency)
- oligohidrosis with hyperthermia (especially in children)
- teratogenicity (oral clefts)
Pharmacokinetics (ADME)
| Absorption | oral; bioavailability ~80%; not significantly affected by food |
| Distribution | 15% protein bound |
| Metabolism | minimal hepatic (20%); induces CYP3A4 at higher doses |
| Excretion | renal (70% unchanged) |
| Half-life | 21 hours |
| Onset | days to weeks |
| Peak | 1–4 hours |
| Duration | 12–24 hours |
| Protein Binding | 15% |
| Vd | 0.6–0.8 L/kg |
Drug Interactions
| Drug / Agent | Mechanism | Severity |
|---|---|---|
| oral contraceptives | CYP3A4 induction at high doses reduces OCP efficacy | moderate |
| valproate | combination increases hyperammonemia risk; both drugs impair urea cycle | moderate |
| metformin | additive metabolic acidosis risk | moderate |
Nursing Considerations
- Hydrate adequately to reduce kidney stone risk; educate patients to drink 2–3 L fluid daily.
- Counsel on word-finding difficulty ('Dopamax') and cognitive effects; reassure that these are dose-dependent and may improve with slower titration.
- For children in hot weather: monitor for decreased sweating and hyperthermia; instruct parents to assess sweat production.
- Check serum bicarbonate periodically; metabolic acidosis occurs in 10–25% of patients; monitor more closely if patient is also on metformin.
Clinical Pearls
- Topiramate produces clinically significant weight loss (5–8 kg average) — a unique advantage in overweight epileptic patients, but limiting in underweight patients.
- The rare acute-onset myopia with angle-closure glaucoma is a medical emergency requiring immediate discontinuation and treatment; patients should be warned to seek emergency care for sudden visual changes, eye pain, or redness.
Safety Profile
Pregnancy avoid
Lactation use-with-caution
Renal Adjustment Required
Hepatic Adjustment Required
TDM Not required
Concordance Terms
Cross-referenced clinical concepts — click any term to see all content where it appears.