levels of doxycycline are reduced and may fall below the accepted minimum inhibitory concentration in patients receiving long-term treatment with phenytoin
. Fosphenytoin, a prodrug of phenytoin
, may interact similarly.
It has been suggested that the doxycycline dosage could be doubled to counteract this interaction. Tetracycline, oxytetracycline and chlortetracycline appear not to interact and may therefore be suitable alternatives.