Drug overdose deaths in the United States just hit a new national record. More than 72,300 Americans died last year as a result of an overdose, according to new estimates from a Centers for Disease Control and Prevention report.

Health officials say the nation’s ongoing opioid epidemic is at the center of the crisis, with heroin, prescription painkillers and synthetic opioids like fentanyl accounting for most of the drug fatalities. They have also reported that drug-related deaths have increased 10 percent since 2016. 

Why are overdose deaths on the rise? There are two reasons, NYMag.com reports. First, synthetic opioids like fentanyl, which can be 50 times stronger than heroin, are increasingly being mixed into street supplies of heroin, cocaine, methamphetamines, benzodiazepines and other drugs, raising the risk of overdose for those who don’t realize what they’re taking. Second, unexpected combinations and dosages of drugs are making drug use riskier even for experienced users.

NYMag.com also notes that deaths from the opioid crisis have long outpaced other high-profile epidemics in U.S. history, including HIV and hepatitis C virus (HCV)-which are also on the rise among opioid users. Overdose deaths this year have also surpassed the number of deaths caused by car crashes in one year: 55,000 in 1972, the deadliest year on record.