Slightly less than 1 percent of anesthesiology residents in the United States have a substance use disorder, according to a new study. The incidence of substance use has been increasing, and relapse rates are not improving, the researchers said.

The study followed 45,000 anesthesiology residents who began their training between 1975 and 2009, HealthDay reports. They found the overall rate of substance abuse was 0.86 percent. Rates were higher at the beginning of the study, and decreased between 1996 and 2002. They began rising again in 2003.

Twenty-eight anesthesiology residents died due to substance abuse during the study period. Among others who abused substances, 43 percent had at least one relapse over the following 30 years, and 11 percent died from a substance use disorder. The most commonly abused substances were intravenous opioids, alcohol, marijuana, cocaine and anesthetics/hypnotics.

The findings are published in the Journal of the American Medical Association.