A new study finds deaths from methamphetamine rose 50-fold between 1999 and 2021 in the United States, HealthDay reports. Many of the deaths also involved heroin or fentanyl.

The study found there were 608 meth-related deaths in 1999, which rose to 52,397 in 2021. Researchers found 61% of fatal meth overdoses also involved heroin or fentanyl.

Study co-author Rachel Hoopsick of the University of Illinois Urbana-Champaign said in a news release that methamphetamine by itself can be deadly, but its toxicity does not appear to have increased in recent years.

“Some of the newer methamphetamine deaths may have involved the unintentional co-use of fentanyl,” she said. “It’s possible that the supply of methamphetamine was adulterated with fentanyl or contaminated by fentanyl at some point in the drug-manufacturing process.”

Linda Richter, vice president for prevention research and analysis at Partnership to End Addiction, who was not involved in the study, noted that meth has become increasingly available, especially in communities of color, and is highly addictive.

“The recent opioid epidemic led to a much-needed focus on opioids, but less attention and resources went to addressing the broader addiction crisis that was simultaneously brewing, especially in relation to stimulants like methamphetamine,” she told HealthDay.