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In March 2012, the French Health Products Safety Agency interrupted the commercialization of di-hydan (phenytoin). It was replaced by diphantoïne (phenytoin sodium) and prescribers were informed that posology was equivalent for both products. We conducted a retrospective study of phenytoinemia and clinical effects comparatively for these two drugs in a population of adult patients with epilepsy admitted in La TEPPE. Forty-four patients were included. Mean age was 47.6 years. Phenytoinemia significantly decreased after substitution (17.14mg/L with di-hydan versus 12.17mg/L with diphantoïne, P<8 10(-6)). Moreover an increase in post substitution posology of diphantoïne was noticed (264.77mg/L with di-hydan versus 274.73mg/L with diphantoïne), although not significant (P=0.11). Increase of seizures was non-significant (P = 0.09). The decrease of phenytoinemia was probably due to the difference of composition between the drugs: a 100mg di-hydan tablet contains 100mg of phenytoin whereas a 100mg diphantoïne tablet contains 92mg. The specific non-linear kinetics of phenytoin reinforces this difference. A prospective study could better evaluate the risk of substituting di-hydan with diphantoïne., (Copyright © 2014 Elsevier Masson SAS. All rights reserved.)