Phenytoin serum levels are raised by
cimetidine (maximum rise of 280% over 3 weeks). Very rarely
bone marrow depression develops on concurrent use. Limited evidence suggests that low (non-prescription) doses of
cimetidine may not interact. Fospheny- toin, a prodrug of
phenytoin, may interact similarly.
Monitor
phenytoin levels and adjust the dose accordingly. Alternatively other H2-
receptor antagonists (ranitidine,
famotidine, nizatidine) do not usually interact, although isolated cases have been reported. Be alert for signs of
phenytoin toxicity (e.g. blurred vision, nystagmus, ataxia or drowsiness) when these
H2-receptor antagonists are first given.