These are the drugs killing the most people in the US

U.S. health officials are unpacking mortality data amid efforts to better understand the country's recent surge in drug overdose deaths, and a new report ranks the drugs that are most frequently involved in these fatalities.

Reflecting what's been described as America's opioid epidemic, the illegal drug heroin and prescription painkiller oxycodone both were at the top of the list for drug-related deaths in 2014, the latest year covered by the study.

But so also were cocaine and the anti-anxiety drug alprazolam, also known by brand names Xanax and Niravam.

Researchers from the Centers for Disease Control and Prevention and the Food and Drug Administration based the data on death certificate information that is typically written by medical examiners or coroners.

"The method was applied to provide a more in-depth understanding of the national picture of the drugs involved in drug overdose deaths," their study says.

The report shows that the number of drug overdose deaths and types of drugs involved in them varied within a short period. From 2010 to 2014, the number of overdose deaths in the U.S. increased by 23 percent, going from 38,329 in 2010 to 47,055 in 2014.

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During that time, the rate of drug overdose deaths involving heroin more than tripled, the report says, while the rate of drug overdose deaths involving methamphetamine more than doubled.

Here is what the rankings looked like in 2010, when heroin and the opioid fentanyl had less of an impact on overdose deaths than in 2014. The researchers did urge caution in comparing numbers across years, however, as the rankings could be affected simply by improved reporting.

The upward trend in overdose deaths continued in 2015, with another CDC report published Friday showing that fentanyl and heroin use appeared to help drive an increase in overdose deaths that year.

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