Deaths from the synthetic opioid fentanyl more than doubled in the past year, new federal data shows.

Fentanyl, an incredibly potent synthetic opioid, is thought to be anywhere from 50 to 100 times more potent than morphine and has commonly been prescribed to manage severe pain after a surgery or debilitating pain from advanced stages of cancer. But in recent years, illegal manufacturing of fentanyl has skyrocketed, and as a result, has led to a sharp spike in the number of drug overdose deaths from synthetic opioids.

The new data comes from the Centers for Disease Control and Prevention. It shows that two-thirds of drug overdose deaths in the United States are related to opioids — and that understanding which drugs are fueling the opioid epidemic in the US is complicated.

The new data shows that nearly half of drug overdoses in the US aren’t isolated to one drug, but rather, involve an increasingly complicated and lethal drug cocktail. And while heroin, an opioid, is a leading cause of drug overdose death in the US, cocaine, a stimulant and part of an entirely different drug family, is the second-leading cause.

Opioids make up six of the top 10 drugs involved in overdose deaths in America

America’s growing problem with drug overdose deaths has long been dominated by opioids. As of 2010, opioid drugs were responsible for a little over 50 percent of all drug deaths, but as you can see in the chart below, that number had grown to 66.4 percent by 2014.

What’s more, the total number of drug overdose deaths from opioids is on the rise, too. In 2010, 17,094 people died from an opioid induced drug overdose, but by 2014 the death toll had nearly doubled to 31,271 deaths from opioids.

Oxycodone (an opioid) was the leading cause of drug overdose deaths in 2010 and 2011 before heroin replaced it as the leading cause of drug overdose deaths in 2012.

By 2014, deaths linked to heroin had more than tripled from 2010 levels, and heroin was responsible for almost a quarter of all drug overdose deaths.

Even more startling than heroin’s accelerated rise is the even more rapid rise of fentanyl, a synthetic and highly potent opioid. The number of fentanyl related deaths more than doubled in a single year, from 1,905 deaths in 2013 to 4,200 deaths in 2014. Fentanyl is now the fifth most commonly found drug in an overdose death.

Not all opioids have contributed to the startling uptick in drug overdose deaths in the US. In fact, some drugs like methadone — a popular opioid used for pain management — have actually declined in prevalence since 2010. The number of deaths related to hydrocodone and morphine abuse has remained relatively stable as well.

It’s not just opioids. Methamphetamine-related drug overdose deaths have doubled since 2010.

While opioids dominate the top 10 drugs found in drug overdose deaths in the US, two stimulants also made the list: cocaine and methamphetamine.

Cocaine has consistently been either the second or third most commonly found drug in a drug overdose death since 2010. But the prevalence of methamphetamine is growing more rapidly.

The number of drug overdose deaths involving methamphetamine has more than doubled from 1,388 deaths in 2010 to 3,728 deaths in 2014. Whereas the number of deaths involving cocaine have only increased by a third.

What’s more, most drug overdoses involving cocaine also involve at least one other drug at the time of death. But for deaths related to methamphetamine, the opposite is true. In drug overdose deaths related to methamphetamine, 55 percent of overdoses were due entirely to meth; no other drugs were present at the time of death.

Nearly half of all drug overdose deaths involved more than one drug

Report authors Margaret Warner and Holly Hedegaard say that one important element of their study was finally being able to measure how many overdose deaths involve multiple drugs.

They found that 48 percent of drug overdose death, or nearly half, reported at least two or more drugs at time of death. This was on par with what the researchers expected, but has shed insight on what combinations of drugs are commonly seen in drug overdose deaths.

For instance, the report found that more than one-third (37 percent) of cocaine deaths also involved heroin. But even more staggering is the number of drugs present in overdose deaths linked to benzodiazepines.

More than 95 percent of drug overdose deaths linked to benzodiazepines (often used to treat anxiety and muscle pain) reported two or more drugs present at the time of death.

Dr. Gregory Davis, the chief coroner in Alabama’s Jefferson County, says that determining which drug is the primary cause of death in drug overdose deaths where multiple drugs are present is a case-by-case situation.

"It’s not the drug with the highest concentration necessarily," said Davis. "What’s an effective concentration depends on the drug. You want to determine which drug had the highest concentration for itself." Davis says that external factors linked to behavior can also be in instrumental in determining which drug or which combination of drugs was responsible.

What’s next in studying the troubling rise in drug overdose deaths

Warner says that her team’s number one research priority is updating their findings with 2015 data. From there, they plan to slice the data across various geographic regions and examine differences in age and racial demographics, as there’s a growing body of literature highlighting the regional and ethnic disparity of drug overdose deaths in the US.

They also plan to take a deeper dive into some of the drugs that didn’t make the top 10 to see if what emerging trends are present there.