Bad reactions to prescription drugs send 700,000 Americans to emergency rooms each year, according to a new federal study providing the most detailed look yet at the problem.

The report, appearing today in the Journal of the American Medical Assn., said drug allergies were the most common bad reaction driving patients to emergency rooms, followed by unintended overdoses. The study excluded suicide attempts.

People older than 65 were more than twice as likely to suffer drug reactions requiring emergency care than younger individuals, with one in six subsequently being admitted to the hospital.

Many of the drugs driving people to emergency rooms have been around for a long time, according to the study. Of the 18 drugs most frequently blamed for causing bad reactions, 16 have been in use for more than 20 years.

Researchers found three drugs – the blood-thinner warfarin, the diabetes drug insulin and the heart medicine digoxin – accounted for one-third of drug-related emergency-room visits by people older than 65.

Insulin and warfarin led the list of drugs causing bad reactions for all patients regardless of age, researchers said, followed by amoxicillin, aspirin and trimethoprim-sulfamethoxazole, an antibiotic.

The study was conducted by the Centers for Disease Control and Prevention, the Food and Drug Administration and the Consumer Product Safety Commission. Researchers used 2004 and 2005 figures from 63 hospitals across the country to derive their national estimates.

Dr. Daniel Budnitz, an epidemiologist with the CDC and lead author of the study, said many of the drugs causing the bad reactions were “good, life-saving drugs.” But the findings of the study show the medical community “needs to do a more careful job monitoring and working with patients,” he said.

Budnitz said elderly patients were nearly as likely to receive emergency treatment for a bad drug reaction as for a motor-vehicle injury. For the population as a whole, bad drug reactions accounted for 2.5 percent of all emergency-room visits for unintended injuries and 6.7 percent of injuries leading to hospitalizations.