Deaths due to drug poisoning have tripled in the last three decades, a new study concludes. The study included poisonings from both illegal and prescription drugs, according to U.S. News & World Report. Prescription drugs make up the majority of drug overdose deaths, the study concluded.

The largest increase occurred in the last decade examined in the study. The researchers at the Centers for Disease Control and Prevention found the percentage of counties with drug poisoning death rates of more than 10 in 100,000 rose from 3 percent in 1999, to 54 percent in 2009. This is the first study to look at drug poisoning rates at the county level in the United States, the article notes. Previous studies have examined rates at the state or national level.

“Mapping death rates associated with drug poisoning at the county level may help elucidate geographic patterns, highlight areas where drug-related poisoning deaths are higher than expected, and inform policies and programs designed to address the increase in drug-poisoning mortality and morbidity,” lead researcher Lauren Rossen said in a statement.

Drug poisoning death rates rose by almost 400 percent in rural areas, and by almost 300 percent in large central metropolitan counties, the study found. Higher rates were found in the Pacific, Mountain, and East South Central regions of the nation. Lower rates were concentrated in the West North Central region, the article notes.

The findings are published in the American Journal of Preventive Medicine.