Food and drug interaction is a major variable in bioavailability of drugs. Isoniazid is the most common antitubercular drug used in India. We studied the effect of standard Indian breakfast and lunch on the bioavailability of isoniazid in a single dose crossover study in normal male volunteers. The standard breakfast and lunch significantly reduced plasma AUC, Cmax and Kabs values of isoniazid. Isoniazid, thus, should not be administered with food.