But the fatal or serious reactions most often reported are only the tip of the iceberg, experts say. "There are tens of millions of milder reactions, some of which are quite damaging to people even though they're medically regarded as minor," says Donald W. Light, a medical sociologist and editor of The Risks of Prescription Drugs, a book that reviews current evidence of medication problems.
Milder symptoms such as drowsiness, sleeplessness, muscle aches, dizziness, nausea and bouts of depression may be more troubling than they are dangerous. Yet, Light says, studies show drugs that affect people's sense of balance or slow their reactions are a major cause of falls and road accidents. Even gastric problems or muscle pain can seriously affect mobility and mood, hampering work, activities and family relationships.
Why do drugs prescribed for a specific health problem trigger other health problems? Errors made by doctors, pharmacists, hospitals — and patients themselves — are a major problem. But even if all errors were avoided, knotty issues remain, including bad interactions among different drugs prescribed for the same patient by different doctors; drugs prescribed for uses that the Food and Drug Administration has not approved; and an imperfect testing system for new drugs, which permits the marketing of medications that only later prove to have harmful side effects.
Moreover, drugs are widely assumed to be designed to target a specific medical issue, but they rarely do. "They have effects on multiple organs," says Schiff. "These are very complex molecules going into very complex organ systems of human bodies."
That's especially true for older people. "As we age, changes occur to all organ systems, and these can affect the way that medication is processed in the body," says Mary Ann Zagaria, a consultant pharmacist in Norwich, N.Y., and a board member of the Commission for Certification in Geriatric Pharmacy. "A drug regimen that was appropriate for a 60-year-old would not necessarily be well tolerated at 70 or 80."
More than 75 percent of Americans age 60 and over take two or more prescription drugs, and 37 percent use at least five, according to the federal Centers for Disease Control and Prevention (PDF). But older people are rarely included in clinical trials, which test a drug's safety and effectiveness.