A person taking a prescription or an over-the-counter medication should consider the food that is in his or her stomach. What a person eats and when he or she ate it may influence the way the medicine works, according to health professionals at New York University Medical Center.

"Antibiotics are a major category of drugs that can be affected by food," said Samuel D. Uretsky, assistant director of pharmacy. "Some antibiotics, including most penicillins, are inactivated by acids and should not be taken with orange or grapefruit juice. They are best taken an hour or two before eating to minimize the effect of stomach acid."An article in an upcoming issue of the center's Health Letter notes that other antibiotics, notably tetracycline, are inactivated by calcium and therefore should not be taken with milk or other dairy foods, nor with foods that are high in iron. "Do not take tetracyclines on an empty stomach," advised Teresita Pia-Geronimo, chief clinical nutritionist. "Since the time required for absorption varies, it is best to take the drug an hour before eating, or two hours after eating."

People who take the type of antidepressant drugs called monoamine-oxidase inhibitors, which include isocarboxazid (Marplan), phenelzine (Nardil) and tranylcypromine (Parnate and others), should be aware of possible interactions with "pressor amines," components of proteins found in numerous foods and beverages.

"The result may be abrupt severe hypertension, headaches, nausea, vomiting and very rapid palpitations. To be avoided: beer, wine, aged cheese, chocolate, avocados, bananas, pickled or dried fish, sour cream, yogurt, salami, chicken liver, meat tenderizers and soy products," she stated.

Excessive sodium loss, as occurs in diarrhea, decreases tolerance to lithium carbonate, a drug used to treat manic-depression disorder, and may increase the risk of toxicity. "Therefore, it is important to maintain adequate salt and water intake," Uretsky noted. "Also, diuretics lower blood levels of lithium."

People may be advised to avoid certain foods for a limited time under specific conditions. For example, green leafy vegetables, fish oils, and green tea contain vitamin K, which helps blood to clot normally. A person taking warfarin or other anticoagulants, the purpose of which is to thin the blood, may be advised not to eat foods that may weaken the drug's actions, such as those high in vitamin K.

"Because high-fiber foods have a laxative action, people eating them generally should not take laxatives, since this could cause depletion of essential minerals," Uretsky cautioned.

He urged that a person taking any drug not make a drastic change in diet without advice.