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Serum phenytoin levels sometimes fluctuate markedly for no apparent reason. In this study, researchers examined variability in phenytoin levels among 56 nursing home residents (mean age, 80) who were taking the drug for seizure control. Phenytoin levels were measured 3 or more times in all patients while they received the same dose for at least 4 weeks, and no patients received other medications that alter phenytoin metabolism. Phenytoin doses were missed, and formulation (capsule vs. suspension) or route of administration (oral vs. gastric tube) was changed, for only a few patients during the study.
Although nearly all patients had at least 1 level within the traditional therapeutic range (10-20 µg/mL), intra-individual variability was cons…