Pop a tetracycline capsule into your mouth, wash it down with milk, and what do you get? If there's an infection lurking in your body, the answer is: no help at all. That's because the calcium in dairy products binds with this antibiotic, rendering it useless.
With tetracycline, as with many other medications, what we don't know about food/drug interactions can sometimes hurt us.
Most of the time, we needn't worry. But the beneficial effects of certain drugs are canceled out by what we eat, while other substances can make us ill when combined with seemingly benign meals.
''The drugs you take can affect the foods you eat. And the foods you eat can affect the drugs you take,'' said R. Rebecca Couris, assistant professor at the Massachusetts College of Pharmacy and Allied Health Sciences in Boston.
Some medications shouldn't be taken with food at all. The new drug Videx, used to treat the HIV infection, loses up to half of its effectiveness if consumed at mealtime because stomach acid breaks down the active ingredients of the drug.
Antibiotics such as penicillin and erythromycin are not as readily absorbed when taken with food and should be consumed two hours before or after a meal. Calcium supplements are less efficiently absorbed at meals.
Other drugs go down better
with food. The so-called NSAIDs - nonsteroidal anti-inflammatory drugs, often prescribed for arthritis, menstrual pain, and other problems - irritate the stomach and cause gastrointestinal pain if consumed alone over a period of time. These include Advil, Motrin, Nuprin and Naprosyn.
The hormone replacement Provera can cause nausea and abdominal pain when taken by itself. Ticlid, a medication used to cut the risk of stroke, also irritates the stomach.
Now comes the rocket science: remembering to avoid certain foods with certain drugs. One of the most toxic combinations comes from mixing the anti-depressants known as MAO inhibitors, such as Marplan and Nardil, with any foods containing the amino acid tyramine. Found in Chianti wine and vermouth, hard cheeses, chocolate, and beef or chicken livers, tyramine can trigger dangerously high blood pressure, heart palpitations, and nausea when consumed with these drugs.
Or take some of the common antihypertensives - better yet, don't take them unless you know what you're doing. Sitting down to a plate of fried rice and shrimp chow mein can undo these good effects if the food is flavored with MSG, which contains sodium. An Alka-Seltzer tablet or a few handfuls of potato chips have the same deleterious results.
Other blood-pressure medications, including ACE inhibitors, cause the body to retain potassium, which can cause cardiac problems and muscular weakness. Sprinkling a salt substitute on your vegetables while taking an ACE inhibitor can dangerously raise potassium levels because many substitutes contain potassium chloride.
Anticoagulants, such as Coumadin, are used to reduce blood clotting. But mixing them with heaping portions of foods rich in vitamin K - spinach, cabbage, cauliflower, brussels sprouts and broccoli - can spell trouble, because Vitamin K produces its own blood-clotting substances. If you're on these drugs, eat just a few helpings of these foods each week.
Alcohol is pre-eminently hazardous with prescription drugs. It aggravates stomach irritation when mixed with NSAIDs. It sets off nausea and vomiting when mixed with certain oral diabetic drugs. And it compounds the drowsiness and sluggish reaction time of barbiturates, anti-anxiety drugs, and over-the-counter antihistamines.
But other bad habits also impair a drug's potency. Smoking cancels out the effects of the asthma drug Theophylline, the anti-anxiety drug Valium, and the analgesic Darvon. In these instances, inhaled cigarette smoke speeds up the liver's enzyme activities, causing the drugs to be metabolized into less active forms.
Caffeine shouldn't be mixed with bronchodilators; in combination, they overstimulate the central nervous system, causing anxiety, restlessness and insomnia. Large quantities of caffeine also flush away water-soluble vitamins such as B-6, B-12 and vitamin C.
''Consumers need to ask questions about their medications,'' said Daniel Hussar, professor at the Philadelphia College of Pharmacy. ''People should know what the active ingredients are in any medication they're taking. And they should know how to use those medications at the most benefit, and with the least risk.''