The mixture, which is being blamed for overdoses in a handful of states, combines several opioids already responsible for thousands of fatal overdoses, including heroin, and synthetic opioids like fentanyl, carfentanil and U-47700.

The gray mixture looks like concrete and is the basis for the drug's nickname.

Fentanyl, a better known lethal synthetic substance, pales in comparison to carfentanil, which is so potent that just a pin drop can kill, according to a report by Time. Produced from morphine, carfentanil is 100 times more powerful than fentanyl and 10,000 times more powerful than morphine, according to the National Institute for Health.

The new mixture has been limited to the Gulf Coast and states like Georgia and Ohio, according to a report by The Associated Press. However, federal drug enforcement officials say they are closely monitoring the potential spread of the drug, the report said.