Phenytoin serum levels can be raised by isoniazid and some patients may develop toxicity. If both rifampicin (rifampin) and isoniazid are given, serum phenytoin levels may fall in patients who are fast acetylators of isoniazid, but may occasionally rise in those who are slow acetylators (said to be about 50% of the population).
It is unlikely that the acetylator status of the patient will be known. Therefore monitor phenytoin levels and adjust the dose accordingly. In some patients the interaction has taken several weeks to develop.
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