Prescription drugs: how to use them safely and effectively.

You see your doctor, get a prescription, have it filled, and take the medicine. What else do you need to know about prescription drugs? Keep reading.

Adverse reactions. Not everyone reacts the same way to medication. One person may experience a reaction to a certain drug, while another person may have no problems at all. Side effects are usually mild -- a slight rash, mild headache, nausea, or drowsiness. Sometimes, however, they are severe -- prolonged vomiting, bleeding, marked weakness, or impaired vision or hearing. These are warning signals that something is wrong. When a reaction is unexpected or severe, a doctor should be consulted immediately.

Drug-drug interactions. Two or more drugs taken at the same time can interact and affect the way one or the other behaves in the body. For example, an antacid will cause a blood-thinning (anticoagulant) drug to be absorbed too slowly, while aspirin will greatly increase the blood-thinning effect of such drugs.

Two drugs with the same effect when taken together can sometimes have an impact greater than would be expected. This is called potentiation. Potentiation can be dangerous, particularly when several drugs designed to depress the central nervous system are involved. Even nonprescription drugs, such as antihistamines for colds, can increase the sedative effects of anesthetics, barbiturates, tranquilizers, and some painkillers.

Food-drug interactions. Food can interact with drugs, making them work faster or slower, or even preventing them from working at all. Here are some examples: Calcium is dairy products impairs absorption of tetracycline, a widely used antibiotic. Citrus fruits or juices containing ascorbic acid speeds up the absorption of iron from iron supplements. Soda pop, and fruit and vegetable juices with high acid content (such as grape, apple, orange, or tomato) cause some drugs to dissolve in the stomach instead of the intestines where they can be more readily absorbed.

The most hazardous food-drug interaction is that of drugs sometimes prescribed for severe depression or high blood pressure and foods containing the substance tyramine. The drugs involved contain monoamine oxidase (MAO) inhibitors. The foods involved include aged cheese, Chianti wine, pickled herring, fermented sausages, yogurt, sour cream, chicken liver, canned figs, bananas, avocados, and foods prepared with tenderizers. Mixing these foods with an MAO inhibitor drug can raise the blood pressure to dangerous levels.

The food-drug interaction can also go the other way. Oral contraceptives, for instance, are known to lower blood levels of folic acid (a member of the vitamin B family), although the depletion is usually not serious enough to cause symptoms. Women who take birth control pills would be wise to include dark-green leafy vegetables in their diet.

Chronic use of antacids containing aluminum can cause phosphate depletion, leading to weakness, malaise, and loss of appetite.

Drugs and alcohol. Chronic use of alcohol can cause changes in the liver that speed up the metabolism of some drugs, including anticoagulants and drugs used for diabetes. They become less effective because they do not stay in the body long enough. Alcohol abuse can also damage the liver so that it is less able to metabolize or process certain drugs. In that case, the drugs stay in the system too long. This is particularly serious when the drugs are phenothiazines (antipsychotic drugs), which can cause further liver damage.

Alcohol depresses the central nervous system (CNS). When taken along with another CNS-depressant drug, alcohol can affect performance skills, judgment, and alertness. If the mixture includes an overdose of barbiturates, diazepam (Valium), or propoxyphene (Darvon), the result can be fatal.

A person who has developed a tolerance to the sedative effects of alcohol may need larger doses of tranquilizers or sleeping pills to get the desired effect. This can lead to an overdose without the person being aware of it.

Drugs and smoking. Women on birth control pills who smoke have an increased risk of heart attack, stroke, and other circulatory disease. Nicotine and other tobacco products speed up the metabolism of certain drugs, for example. Thus, smokers may need larger-than-normal doses of these drugs. When they stop smoking, dosage of these drugs may have to be changed. Smoking can also affect certain diagnostic tests, such as red and white blood cell counts and blood clotting time determinations.

Drugs and laboratory tests. Drugs can affect the results of clinical laboratory tests. For example, excess use of laxatives can affect tests to determine calcium or bone metabolism. Penicillin can result in false readings of protein in the urine, a sign of kidney disease. Large doses of vitamin C can produce false results in a urinary glucose test for diabetes.

What you should tell your doctor. Because of these side effects and drug interactions, it's important that you tell your doctor if you:

* have had allergic reactions to drugs or foods, such as rashes or headaches.

* are taking any medication on a regular basis (such as contraceptives, insulin), or if you routinely use any non-prescription drugs.

* are being treated for a different condition by another doctor.

* are pregnant or breast-feeding.

* have diabetes or kidney or liver disease.

* are on a special diet or are taking vitamin and mineral supplements.

* use alcohol or tobacco.

What you should ask your doctor. To get the most out of your medicine, you should ask your doctor:

* What is the name of the medicine? Write it down so you won't forget.

* What is the medicine supposed to do? (Make the pain go away? Get to the cause of the pain? Reduce fever? Lower blood pressure? Cure infection?)

* What side effects might occur?

* How should you take the medicine? Does "three times a day" mean morning, noon, and night? Should you take it before meals, with meals, or after meals? If the directions say "every six hours," do you have to get up during the night to take the medicine on time?

* How long should you take the medicine? (If you stop just because you feel good, the symptoms and the disease may recur.)

* Are there other medicines you should not take while taking this one?

* Are there any foods or beverages you should avoid?

* Should you avoid alcoholic beverages while taking this medication?

* Can this prescription be refilled without an appointment, or does the doctor need to see you again?

What's on your prescription. The first word on the prescription is the name of the drug. Next is the dosage form (liquid, capsules, tablets) and the strength (such as 250 milligrams). Next is the amount you will get (15 capsules, 5 fluid ounces), followed by the directions for use. These are often abbreviations of Latin words. For instance, ter in die (written t.i.d.) means three times a day. The pharmacist will translate this information on the label of the medicine container. The prescription form will also indicate how many times the prescription can be refilled. It's a good idea to recheck the prescription before you leave the doctor's office. If there is anything you don't understand, ask about it.

Information for patients. Detailed information for patients does not have to accompany most prescription drugs. But for a few, such as contraceptives and estrogens, the Food and Drug Administration does require a leaflet or brochure that tells about the benefits and risks of these products. If there is such a brochure with a drug prescribed for you, be sure to read it carefully; if you have questions, ask your doctor or pharmacist. Nonprescription drugs must include on their labels information about when and how to take the product, possible side effects, or drug interactions. Always read these labels before taking the medicine.

Drug costs: brand versus generic. When a new drug is discovered, it is patented and given a brand, or trade, name. The drug company has exclusive rights to that product for 17 years. When the patent expires, other companies may manufacture that drug under another brand name or the drug's generic or chemical name. These drugs are often less expensive than the original drug. Almost all states now permit pharmacists to dispense a generic drug instead of a brand-name product if the doctor approves. However, not all drugs are available in generic form, and not all generic drugs are therapeutically equivalent -- that is, behave in the same way in the body. The FDA publishes lists of drugs that identify which ones can be substituted safely. The next time your doctor writes a prescription for you, ask if there is an approved generic equivalent of the drug.

Prescription drugs are powerful chemicals. Along with their benefits, they have a potential for harm. But warnings about their possible effects are only as good as a person's willingness to heed them.

Harold L. Hamilton, R.Ph., is a pharmacist consultant at Grady Memorial Hospital, Atlanta, Georgia.