some tolerate levels above the ‘normal’ range without clinical toxicity. National guidance is available on when to check levels; indications include managing drug interactions, confirming clinical suspicion of toxicity and serial measurement to monitor compliance in difficult cases.5 Phenytoin is the exception to this rule; due to its zero order kinetics small adjustments in dose can lead to large fluctuations in serum concentration meaning blood levels are advised, usually one week following dose changes. However, these levels must always be interpreted in their clinical context.