- Drugs whose efficacy is impaired by phenytoin include: anticoagulants, corticosteroids, coumarin, digitoxin, doxycycline, estrogens, furosemide, oral contraceptives, rifampin, quinidine, theophylline, vitamin D.
<phenytoin, effect, anticoagulants>, <phenytoin, effect, corticosteroids>, <phenytoin, effect, coumarin>, <phenytoin, effect, digitoxin>, <phenytoin, effect, doxycycline>, <phenytoin, effect, estrogens>, <phenytoin, effect, furosemide>, <phenytoin, effect, contraceptives>, <phenytoin, effect, rifampin>, <phenytoin, effect, quinidine>, <phenytoin, effect, theophylline>, <phenytoin, effect, vitamin D>