Interaction of furosemide and phenytoin in the rat

The interaction of phenytoin and furosemide was examined in rats. After 2 or more weeks of pretreatment with phenytoin, the diuretic activity of furosemide given orally was decreased. This was due primarily to a generalized decrease in absorption from the gastrointestinal tract. Following intravenous administration of furosemide to phenytoin pretreated rats, a decrease in diuretic activity also occurred. Excretion of furosemide was not altered. Thus, phenytoin pretreatment would also appear to interfere directly with the renal action of furosemide.