Four medications or medication groups — used alone or together — were responsible for two-thirds of emergency hospitalizations among older Americans.
At the top of the list was warfarin, also known as Coumadin, a blood thinner. It accounted for 33 percent of emergency hospital visits.
Insulin injections were next on the list, accounting for 14 percent of emergency visits.
Aspirin, clopidogrel and other antiplatelet drugs that help prevent blood clotting were involved in 13 percent of emergency visits.
And just behind them were diabetes drugs taken by mouth, called oral hypoglycemic agents, which were implicated in 11 percent of hospitalizations.
All these drugs are commonly prescribed to older adults.
One problem they share is a narrow therapeutic index, meaning the line between an effective dose and a hazardous one is thin.
Every year, about 100,000 people in the United States over age 65 are taken to hospitals for adverse reactions to medications.
About two-thirds end up there because of accidental overdoses, or because the amount of medication prescribed for them had a more powerful effect than intended.