A bigger share of U.S. drug overdose deaths are being caused by methamphetamine, government health officials reported.

The number of fatal overdoses involving meth more than tripled between 2011 and 2016, the Centers for Disease Control and Prevention reported Dec. 12. The percentage of overdose deaths involving meth grew from less than 5 percent to nearly 11 percent.

Meth is not the main killer among illicit drugs. Fentanyl was involved in the highest percentage of fatal overdoses in 2016, followed by heroin and cocaine. Meth was fourth. But it was only eighth as recently as 2012.

It's not clear why meth overdoses are growing, but some people who had been abusing opioid pain pills or shooting heroin have turned to meth, a stimulant, to offset the downer effects of those drugs, said Theodore Cicero, a Washington University researcher who has studied the rise of meth use among people who use opioid drugs.

Meth is most often smoked, snorted or injected. Chronic use has been tied to sleeplessness, paranoia and other mood and mental health disorders. And the effect of high doses on the body can include convulsions, rapid heart rate and other heart problems.

“It's a very dangerous drug to mess around with,” Cicero said.

Meth has become more prevalent in certain states, including West Virginia, which has the nation's highest overdose death rate.

Brandon Kirk, a 31-year-old former pharmacy school student, abused prescription opioid painkillers for years but went into recovery earlier this year. In a recent interview, he said it's become increasingly difficult to get opioid painkillers in that part of the state.

“It's flooded with methamphetamine,” he said.

The CDC report looked at death certificates on 64,000 U.S. overdose deaths in 2016 and compared them with the five previous years. Many of the people who died had used multiple drugs — fentanyl was often in the mix.

Fewer than one-third of the heroin deaths involved heroin alone, and about a quarter of cocaine deaths were cocaine alone. About half of the meth death certificates listed that drug only.