(CNN) While there's early evidence that the explosive rate of opioid deaths has started to slow, opioids killed more than 49,000 people in the United States in 2017, according to preliminary data. A new study reveals which part of the country has been affected the most by the ongoing epidemic.

In a study of opioid deaths from 1999 to 2016, "we found that, in general, opioid mortality is skyrocketing," said Mathew Kiang , a postdoctoral research fellow at Stanford University's Center for Population Health Sciences

More specifically, the study, published Friday in the journal JAMA Network Open , found that the mortality rate from synthetic opioids in 28 states more than doubled every two years from 1999 to 2016. The District of Columbia saw the greatest increase in its opioid mortality rate, which more than tripled every year since 2013.

Synthetic opioids are manmade drugs such as fentanyl, as opposed to semi-synthetic opioids such as hydrocodone and oxycodone, or natural opioids such as codeine and morphine.

Fentanyl is up to 50 times more powerful than heroin, and just ¼ of a milligram can be deadly. For comparison, a standard low-dose aspirin is 81 mg. If you were to cut that tablet into 324 pieces, one of those pieces would equal ¼ milligram.