Phenytoin side effects

Clinical Pharmacology and Toxicology

A 26 year old hairdresser develops has a single tonic clonic fit whilst at work, unrelated to any substance misuse or head injury. She is sexually active. She takes the oral contraceptive pill. Which of the following statements are true?

Please select UP TO 2 answers.

Antiepileptic medicine (assuming no obvious contraindications) is always advised following a fit in these circumstances
Phenytoin will reduce the efficacy of her contraception
Phenytoin will increase the efficacy of her contraception
Phenytoin will have no effect on the the efficacy of her contraception
Phenytoin is potentially tetratogenic
She will be legally unable to drive for the next six months only
In common with most other drugs phenytoin is metabolised by “first order metabolism”


Phenytoin is an enzyme inducer and will therefore affect her oral contraceptive pill making it potentially less effective.

Most antiepileptics are p[potentially tetratogenic. You can’t Drive (DVLA) for 1 year after an unprovoked seizure or “first fit”.

After a “first fit” antiepileptic medication is not always indicated. Many physicians prefer to adopt a “watch and wait” approach pending the investigations etc. If no underlying precipitant is found, general advice and follow up can be undertaken.

Phenytoin like alcohol is metabolised by zero order kinetics. This will be dealt with in a separate question.