People over 65 faced the greatest risks.
The results, from 2004-5, represent the first two years of data from a national surveillance project on outpatient drug safety. The project was developed by the federal Centers for Disease Control and Prevention, the Food and Drug Administration and the United States Consumer Product Safety Commission.
The study is to be published Wednesday in the Journal of the American Medical Association.
“This is an important study because it reinforces the really substantial risks that there are in everyday use of drugs,” said Bruce Lambert, a professor at the college of pharmacy at the University of Illinois, Chicago.
The study’s authors and other experts agreed that 700,000 was probably a low estimate because bad drug reactions were likely to be misdiagnosed.
The medicines most commonly implicated in the reactions included insulin for diabetes; warfarin, an anticoagulant, and amoxicillin, an antibiotic used for many infections.
“These are old drugs which are known to be extremely effective,” said Professor Lambert, who was not involved in the research. “We could not and would not want to live without them. But you’ve got to get the dose exactly right. Variations, especially on the high side, are really dangerous.”
People 65 and older faced more than double the risk of requiring emergency room treatment and were nearly seven times more likely to be admitted to the hospital than younger patients.
The study’s database included 63 nationally representative hospitals that reported 21,298 bad drug reactions among adults and children treated in emergency rooms during the two-year period.
The tally is based on what emergency room doctors said were complications from using prescription drugs, over-the-counter medicines, dietary supplements or herbal treatments. The researchers said it translated to 701,547 complications nationwide each year.
“Experts had thought that severe outpatient drug events were common, but no one really had good numbers” until now, said the study’s main author, Dr. Daniel Budnitz.