The daily educational pearl – phenytoin

Phenytoin when used as a loading dose in ED:

– loading dose: 18 mg /kg, max 1.2 g

– patients need to be cardiac monitored due to the risk of arrhythmias

– infusion rate is max 50 mg/min (or 1 mg/kg/min) as the propylene glycol it contains can cause arrhythmias and hypotension

– should not be mixed with glucose solutions (as it will precipitate)



– known allergy to phenytoin

– 2nd or 3rd degree AV block

– seizure activity secondary to toxic ingestion



– arrhythmias / hypotension

– toxic epidermic necrolysis / Stevens-Johnson syndrome

– if accidentally given intraarterially or subcutaneously, it can cause extensive soft tissue injury due to the alkaline pH

Toxicity (chronic use or acute ingestion):

– correlated with serum levels:

20 – 30 mg/L – nystagmus

30 – 50 mg/L – ataxia (NB check phenytoin levels on all patients on chronic treatment who present with ataxia)

> 50 mg/L – coma

– symptoms resolve slowly over 2 – 4 days

– there is no specific antidote – supportive care is key

– massive overdoses can cause seizures (paradoxically) / hypernatraemia / hyperglycaemia / permanent cerebellar injury