(FORTUNE Magazine) – You get a prescription, you pop your pills, and in a few days you feel better. Sounds easy. But taking medicine is not as simple as it seems. And we'd do a much better job of treating what ails us if we'd take drugs as they're prescribed.

Example: Medications that lower cholesterol can prevent heart attacks. Even so, studies show that 15% to 50% of people who have been put on cholesterol-lowering drugs aren't taking them a year later. Generally speaking, people complete about half the doses of all medications they are prescribed. They forget, it's not convenient, the drug costs too much, whatever.

The high price of drugs prompts some patients to take them less often to make them last longer. That's asking for trouble. And going to Canada or Mexico to buy drugs or turning to mail-order companies on the Net may save money, but beware--your neighborhood pharmacist won't be around to help if there's a problem.

Generic brands are less expensive and a reasonable substitute in most cases, but not all. Different brands sometimes have slightly different potencies. A patient of mine was on thyroid medication, but his thyroid blood tests were out of whack; it turns out he had switched to a different but not equivalent preparation. Once he was back on his old brand, his tests promptly reverted to normal.

A 20-milligram tablet often costs the same as a ten-milligram pill, so some people try to save money by splitting a pill in half. A recent study reported that Viagra, Lipitor, and nine other popular drugs could be safely split. This is easy when pills are scored (notched), and easier yet if you use a pill cutter, a guillotine-like device that sells for about $4. But don't try to cut in half a capsule, a pill that's enteric-coated (to be gentle on the stomach), or extended-release medication.

Side effects can be an issue. Sorbitol, sometimes added to liquid medications, can cause diarrhea in some people. Another problem: allergies. We're not born allergic to medications, but we acquire allergies throughout life. Check with your health-care provider if you suspect a side effect or allergic reaction.

The pharmaceutical companies are now into direct advertising, and we're regularly treated to TV commercials about how a medication changed a celebrity's life. The ads probably do increase awareness about a medication for a specific disease, but excessive enthusiasm for a magic cure can be counterproductive. A physician's judgment is still necessary to determine whether a drug is appropriate.

Obviously, medications should be kept far away from children--and that includes vitamins. (Many kids get iron poisoning from their parents' iron supplements.) Most medications are out if you're pregnant, and women who are breast feeding should absolutely consult with their physician or pharmacist before starting a drug, even an over-the-counter one.

Some meds don't mix. People on the blood thinner warfarin shouldn't take aspirin--it increases the risk of bleeding. St. John's wort, an herbal supplement used for mild depression, can lower the concentration of certain prescription medications, including digoxin (a drug for heart failure) and oral contraceptives. And taking Tylenol with alcohol, even if you're only a social drinker, can virtually destroy your liver.

So what's my prescription? Read the package insert, use common sense, and ask your pharmacist or physician for advice when unsure. Drugs can be life-saving when used properly. When they're not, they can cause disease rather than treat it.

Donald D. Hensrud, M.D., is director of the Mayo Clinic Executive Health Program. For more on this topic, go to mayoclinic.com. Mayo Clinic offers Executive Health Programs in Jacksonville; Rochester, Minn.; and Scottsdale, Ariz.