The consequences of drug noncompliance may be serious in older patients. Estimates of the extent of noncompliance in the elderly vary, ranging from 40% to a high of 75%. Three common forms of drug treatment noncompliance are found in the elderly: overuse and abuse, forgetting, and alteration of schedules and doses. Some older patients who are acutely ill may take more than the prescribed dose of a medication in the mistaken belief that more of the drug will speed their recovery. Such overuse has clearly been associated with adverse drug effects. Forgetting to take a medication is a common problem in older people and is especially likely when an older patient takes several drugs simultaneously. Data suggest that the use of three or more drugs a day places elderly people at particular risk of poor compliance. The use of at least three drugs, and often more, is common in the elderly, with estimates of as many as 25% of older people taking at least three drugs. Averages of drug use among elderly hospitalized patients suggest that eight drugs taken simultaneously may be typical. Problems may also arise when dementia or depression is present, which may interfere with memory. The most common noncompliant behavior of the elderly appears to be underuse of the prescribed drug. Inappropriate drug discontinuation, furthermore, may occur in up to 40% of prescribing situations, particularly within the first year of a chronic care regimen. As many as 10% of elderly people may take drugs prescribed for others; more than 20% may take drugs not currently prescribed by a physician.(ABSTRACT TRUNCATED AT 250 WORDS)