Foods nourish your body. Medicines cure (or relieve) what ails you. You’d think the two would work together in perfect harmony to protect your body. Sometimes they do. Occasionally, however, foods and drugs square off like boxers slugging it out in the ring.

The drug keeps your body from absorbing or using the nutrients in food, or the food (or nutrient) prevents you from getting the benefits of certain medicines. The medical phrase for this sad state of affairs is adverse interaction. This article describes several adverse interactions and lays out some simple strategies that enable you to short-circuit them.

When you eat, food moves from your mouth to your stomach to your small intestine, where the nutrients that keep you strong and healthy are absorbed into your bloodstream and distributed throughout your body.

Take medicine by mouth, and it follows pretty much the same path from your mouth to your stomach, where it’s dissolved and passed along to the small intestine for absorption. Nothing is unusual about that.

The problem, however, arises when a food or drug brings the process to a screeching halt by behaving in a way that stops your body from using either the drug or the food. Many possibilities exist:

• Some drugs or foods change the natural acidity of your digestive tract so that you absorb nutrients less efficiently. For example, your body absorbs iron best when your stomach is acidic. Taking antacids reduces stomach acidity — and iron absorption.

• Some drugs or foods change the rate at which food moves through your digestive tract, which means that you absorb more (or less) of a particular nutrient or drug. For example, eating prunes (a laxative food) or taking a laxative drug speeds things up so that foods (and drugs) move more quickly through your body and you have less time to absorb medicine or nutrients.

• Some drugs and nutrients bond (link up with each other) to form insoluble compounds that your body can’t break apart. As a result, you get less of the drug and less of the nutrient. The best-known example: Calcium (in dairy foods) bonds to the antibiotic tetracycline so that both zip right out of your body.

• Some drugs and nutrients have similar chemical structures. Taking them at the same time fools your body into absorbing or using the nutrient rather than the drug. One good example is warfarin (a drug that keeps blood from clotting) and vitamin K (a nutrient that makes blood clot). Eating lots of vitamin K-rich leafy greens counteracts the intended effect of taking warfarin.

• Some foods contain chemicals that either lessen or intensify the natural side effects of certain drugs. For example, the caffeine in coffee, tea, and cola drinks reduces the sedative effects of antihistamines and some antidepressant drugs but increases the nervousness, insomnia, and shakiness common with some diet pills and cold medications containing caffeine or a decongestant (an ingredient that temporarily clears a stuffy nose).

When you pick up an over-the-counter drug or get a new prescription, read the label. Let me repeat that: Read. The. Label. Warnings and interactions often are right there on the package. If they’re not, ask your doctor or pharmacist whether you need to avoid any specific foods while you’re taking the drug. Go on, now — ask.

Or you can plunk down a few bucks for your very own copy of The Essential Guide to Prescription Drugs 2006 (New York: Harper Collins, 2006), the latest edition of the book listed as one of the sources for the tables in here. The book is affordable, readable, and reliable. Go for it.

Not every food and drug interaction is an adverse one. Sometimes a drug works better or is less likely to cause side effects when you take it on a full stomach. For example, aspirin is less likely to upset your stomach if you take the painkiller with food, and eating stimulates the release of stomach juices that improve your ability to absorb griseofulvin, an antifungus drug. Table below lists some drugs that may work better when your stomach is full.

Purpose	Drug
Analgesics (painkillers)	Acetaminophen Aspirin Codeine Ibuprofen Indomethacin Mefenamic acid Metronidazole Naproxen/naproxen sodium
Antibiotics, Antivirals, Antifungals	Ethambutol Griseofulvin Isoniazid Ketoconazole Pyrimethamine
Antidiabetic Agents	Glipizide Glyburide Tolazamide Tolbutamide
Cholesterol-Lowering Agents	Cholestyramine Colestipol Lovastatin Probucol
Gastric Medications	Cimetidine Ranitidine

James J. Rybacki, The Essential Guide to Prescription Drugs 2002 (New York: Harper Collins, 2001)

Don’t guess about drugs and food. Every time you take a pill, read the package label or check with your doctor/pharmacist to find out whether taking the medicine with food improves or reduces its ability to make you better. Or thumb through your brand-new copy of The Essential Guide to Prescription Drugs.