Going into surgery can be an anxiety-producing experience. Now the results of a new study may make patients even more frightened about surgical procedures.

The study published earlier this week in the journal Anesthesiology found that about half of the 277 surgeries tracked had issues with drug effects, according to Bloomberg. In one-third of the cases, patients experience ill effects either from medication errors or unintended reactions to drugs.

The observational study was conducted by the anesthesiology department of the Massachusetts General Hospital, considered one of the country’s top hospitals. Previous studies based on self-reporting by surgeons found few such errors.

In 1999, the  Institute of Medicine recognized medical errors as a leading cause of death, with higher mortality rates than breast cancer or auto accidents. Checklists for surgeons and electronic prescribing systems have been implemented in hopes of reducing deaths related to medical errors.

But prior to the study this week, there have been few systemic studies of mistakes from surgeries.

While problems related to the mistakes were common, no patients tracked in the study died from medical errors, and only three cases were considered life threatening.

Still, with medication errors occurring in one of every twenty drug administrations, the study suggests hospitals need to do more to cut down on mistakes.