The latest numbers from the Centers of Disease Control and Prevention show that one in four drug overdoses in 2015 was related to heroin.

In 1999, just 6% of all overdoses were related to the drug.

When looking at overdoses overall, opioid-related deaths represented the majority. In 2015, overdoses involving opioids represented 73% of all overdose deaths, a significant jump from 57% in 2010.

Opioids include heroin as well as drugs with a similar chemical structure, such as oxycodone and illicit synthetics like fentanyl.

Dr. Holly Hedegaard of the National Center for Health Statistics, who co-authored the study, also noted that this was the first time the number of overdose deaths in the United States exceeded 50,000.

In 2010, there were 38,329 overdose-related deaths, and by 2015, that number had climbed to 52,404. By comparison, in 2015, there were 36,252 total firearm-related deaths across the country. As with heroin, there was also a significant increase in deaths involving synthetic opioids such as fentanyl, the drug that was blamed for pop star Prince's death. In 2010, these types of drugs were involved in just 8% of all overdose deaths, and by 2015, they were involved in 18% of all overdose deaths.

While there were increases in heroin and synthetic drug-related deaths, there was a drop in overdose deaths involving natural and semisynthetic opioid analgesics, including prescription drugs like oxycodone and hydrocodone. Although these drugs were involved in 29% of drug overdose deaths in 2010, they represented 24% of all drug overdose deaths in 2015.

This shift in numbers may in part be due to a change in user habits, with some starting out with prescription drugs and moving on to heroin because of cost and crackdowns on illegal use of prescription drugs. However, Dr. Andrew Kolodny, co-director of Brandeis University's Opioid Policy Research Center, said that switching is only part of the story. "Starting in 2011, overdoses involving heroin has really skyrocketed. There's a really good chance the increase involving heroin has to be involved with fentanyl," he said.