Every drug has the potential to do harm (an adverse drug reaction) as well as good. When doctors consider prescribing a drug, they must weigh the possible harms against the expected benefits. Use of a drug is not justified unless the expected benefits outweigh the possible harms. Doctors must also consider the likely outcome of withholding the drug. Potential benefits and harms can never be determined with mathematical precision.
When assessing the benefits and risks of prescribing a drug, doctors consider the severity of the disorder being treated and the effect it is having on the person's quality of life. For example, for relatively minor disorders—such as coughs and colds, muscle strains, or infrequent headaches—only a very low risk of adverse drug reactions is acceptable. For such symptoms, over-the-counter drugs are usually effective and well tolerated. When used according to directions, over-the-counter drugs for treating minor disorders have a wide safety margin (the difference between the usual effective dose and the dose that produces severe adverse drug reactions). In contrast, for serious or life-threatening disorders (such as a heart attack, stroke, cancer, or organ transplant rejection), a higher risk of a severe adverse drug reaction is usually more acceptable.
Individuals may have different perspectives on quality of life and the risks they are willing to assume. For example, some people are more willing than others to accept the adverse effects of certain cancer chemotherapy in return for a slight chance of prolonging life. People also differ on how great a possibility of risk they are willing to accept. For example, a 1 in 50 chance of having serious bleeding from a drug may be unacceptable to some people but seem reasonable to others.
(See also Medical Treatment Decisions.)