Anticoagulants were among the drugs that caused the most adverse drug events (ADEs) during hospital stays in 2011, according to a report published by the federal Agency for Healthcare Research and Quality (AHRQ).

“Overall, ADEs are the most common nonsurgical adverse events that occur in hospitals,” wrote authors Audrey J. Weiss, PhD, and Anne Elixhauser, PhD, of AHRQ.

Using data from 20,172,966 discharges from 32 states in 2011, agency researchers found that steroids, antibiotics, opiates and narcotics, and anticoagulants were the top four causes of ADEs. ADEs occurred in about eight in 1,000 adults 65 and older. Rates were highest among the oldest adults, those with Medicare as their primary insurance and those in Midwestern U.S. hospitals.

Anticoagulants led to ADEs in 6.7 per 10,000 discharges and were also more common in hospitals that were urban and non-teaching, private and nonprofit and large in terms of the number of beds. The rate of ADEs caused by these drugs was 20 percent lower in females.

Approximately 380,000 to 450,000 patients experience ADEs during hospital stays every year, the authors wrote.

The data are published in the October Healthcare Cost and Utilization Project report.