Introduction

Topiramate was developed as an antiepileptic and is used alone or as adjunctive treatment in generalised tonic-clonic seizures.

Mechanism of action

There appear to be multiple mechanisms of action:
  • blocks voltage-gated Na+ channels
  • increases GABA action
  • carbonic anhydrase inhibition: this results in a decrease in urinary citrate excretion and formation of alkaline urine that favours the creation of calcium phosphate stone

Adverse effects

Adverse effects of topiramate include:
  • Reduced appetite and weight loss
  • Dizziness
  • Paraesthesia
  • Lethargy and poor concentration
  • Rare but important: acute myopia and secondary angle closure glaucoma

Contraindications

Topiramate is an inducer of the P450 enzyme CYP3A4. This may result in hormonal contraception being less effective. As a result the Faculty of Sexual and Reproductive Health (FSRH) suggests the following for patients taking topiramate:
  • Combined oral contraceptive pill and progestogen-only pill: UKMEC 3 (disadvantages outweigh advantages):
  • Implant: UKMEC 2 (advantages generally outweigh the disadvantages)

The injection (Depo-Provera) and intrauterine system are not affected by topiramate.