Medications are prescribed and taken precisely because they have certain desirable effects on the human body. The desirable reaction caused by a medication is known as the drug's activity, or therapeutic effect. But medications sometimes have undesirable effects as well. These unwanted effects are referred to as side effects, adverse reactions, or, in some rare cases, lethal effects.

Even if you experience minor side effects, it is very important that you continue to take your medication exactly as it was prescribed. You should take the full dose at the appropriate times throughout the day for the length of time prescribed by your doctor. It is simply not appropriate to take a lesser amount of medication to avoid side effects or because your condition appears to be improving. It is not valid to assume that taking half of the regular dose will provide half of the therapeutic effects. Indeed, a smaller dose may not provide any benefit whatsoever.

Some side effects are expected and unavoidable, but others may surprise the doctor as well as the patient. Such unexpected reactions may be due to an individual's particular response to the drug.

Side effects generally fall into one of two major categories -- those that are obvious and those that can be detected only through laboratory testing. Discussion between you and your doctor about your medication should not be limited to the most easily recognized side effects. Other, less obvious, side effects may also be harmful, and you should be sure you understand what they are.

If you know a particular side effect is expected from a specific drug, you can relax a little. Most expected side effects are temporary and need not cause alarm. You'll merely experience discomfort or inconvenience for a short time.

For example, you may become drowsy after taking an antihistamine or develop an upset stomach after taking the antibiotic erythromycin. Of course, if you find minor side effects especially bothersome, you should discuss them with your doctor, who may be able to prescribe another drug or at least assure you that the benefits of the drug you are taking far outweigh its side effects.

Sometimes side effects can be minimized or eliminated by changing your dosage schedule or taking the drug with meals. Be sure to consult your doctor or pharmacist if you have questions.

Some side effects signal a serious, perhaps even dangerous, problem. When a new medication is prescribed for you, you should ask your doctor what side effects might occur that would signal the need for immediate attention.

The following sections address obvious side effects, broken down by the body parts or systems affected by medication. The last section discusses those side effects that are more subtle or discernable only through laboratory testing. With subtle side effects, you may not be aware of any symptoms at all, or you may notice only slight ones. Therefore, your doctor may want you to have periodic laboratory tests or examinations to ensure that no subtle damage is occurring while you are on certain medications.

In the next section, let's start by reviewing the potential side effects to the circulatory system.