Opioid overdose deaths have increased more than six-fold since the 1970s, and an estimated 2.5 million Americans abuse this class of drugs. But the deadliest killer is not heroin. Synthetic opioids, which can kill much faster than heroin, have fueled the epidemic. Here's what they are:

• "Grey death": The newest street drug is a dangerous opioid cocktail that is 5,000 times as potent as heroin (CNN)
• Carfentanil: This elephant tranquilizer is as powerful as "grey death" (National Affairs)
• Fentanyl: The most commonly found synthetic opioid is 50 times as deadly as heroin (National Affairs)
• Furanyl fentanyl: This version of fentanyl is 10 times as potent as heroin (USA Today)
• U-47700: This research chemical, still legally available, is eight times as potent as heroin (NBC)

FiveThirtyEight noted that available data on drug abuse, which would help policymakers and researchers combat the drug epidemic, has dropped drastically, potentially making the problem worse.