Doctor error may be the cause of nearly 500,000 preventable drug side effects each year in non-hospitalized elderly patients, according to a new study published Wednesday in the Journal of the American Medical Association. Conducted by researchers at the University of Massachusetts Medical School and Boston's Brigham and Women's Hospital, the study examined data from 30,000 Medicare patients in New England.

According to the study, nearly 90,000 avoidable medication reactions, such as unusual bleeding in patients on blood thinners, are life-threatening or fatal. Researchers blame inadequate patient monitoring and harmful drug interactions for a majority of the side effects. The study is believed to be one of the first to analyze preventable drug side effects in non-hospitalized elderly patients.