The main goals of drug development are effectiveness (efficacy) and safety. Because all drugs can harm as well as help, safety is relative. The difference between the usual effective dose and the dose that produces severe or life-threatening side effects is called the margin of safety. A wide margin of safety is desirable, but when treating a dangerous condition or when there are no other options, a narrow margin of safety often must be accepted. If a drug's usual effective dose is also toxic, doctors do not use the drug unless the situation is serious and there is no safer alternative.
The most useful drugs are effective and, for the most part, safe. Penicillin is such a drug. Except for people who are allergic to it, penicillin is virtually nontoxic, even in large doses. On the other hand, barbiturates, which were once commonly used as sleep aids, can interfere with breathing, dangerously lower blood pressure, and even cause death if taken in excess. Newer sleep aids such as temazepam and zolpidem have a wider margin of safety than barbiturates do.
Designing effective drugs with a wide margin of safety and few side effects cannot always be achieved. Consequently, some drugs must be used even though they have a very narrow margin of safety. For example, warfarin, which is taken to prevent blood clotting, can cause bleeding, but it is used when the need is so great that the risk must be tolerated. People who take warfarin need frequent checkups to see whether the drug is causing the blood to clot too much, too little, or appropriately.
Clozapine is another example. This drug often helps people with schizophrenia when all other drugs have proved ineffective. But clozapine has a serious side effect: It can decrease the production of white blood cells, which are needed to protect against infection. Because of this risk, people who take clozapine must have their blood tested frequently as long as they take the drug.
To help ensure that their treatment plan is as safe and effective as possible, people should keep their health care practitioners well informed about their medical history, drugs (including over-the-counter drugs) and dietary supplements (including medicinal herbs (see Medicinal Herbs and Nutraceuticals: Overview of Medicinal Herbs and Nutraceuticals) that they are currently taking, and any other relevant health information. In addition, they should not hesitate to ask a doctor, nurse, or pharmacist to explain the goals of treatment, the types of side effects and other problems that may develop, and the extent to which they can participate in the treatment plan.
Last full review/revision May 2007 by Eva M. Vivian, PharmD