Synthetic opioids continue to morph into new and deadly forms. Take the latest warning from law enforcement in the Southeast about fatal overdoses from a compound dubbed “gray death.”

Consumed in rock or powder form, with the appearance of concrete mixing powder, this newly identified drug cocktail combines heroin, fentanyl and U-47700 and has been linked to at least four overdose deaths in Georgia  and Alabama, according to a law enforcement document obtained by NBC News.

Fentanyl and U-47700 are more powerful than heroin, and together, the drug combo is many times more powerful than heroin alone. Because the chemistry of the drug varies from batch to batch, it’s unpredictable how it will affect the user.

Last year, two teen boys died from ingesting U-47700 in Park City, Utah after it was delivered — legally — to their homes via the mail. Also known as “Pink,” U-47700 is eight times stronger than heroin, and is part of a family of deadly synthetic opioids, all of them more powerful than heroin, that includes ifentanyl, carfentanil and furanyl fentanyl.

Total opioid overdose deaths in the U.S. nearly quadrupled between 1999 and 2014, rising from 8,050 to 28,647, according to statistics from the U.S. Centers for Disease Control. The portion of those deaths caused by synthetic opioids rose almost twice as fast, from just 730 in 1999 to 5,544 in 2014.