It creates antibodies that block the ‘high’ of the drug and enable it to be processed safely by the kidneys before leaving the body. Fentanyl is a synthetic opioid. It was developed to ease pain in cancer patients, but because of its high strength, it’s sold as a street drug. Cases of fentanyl overdoses are growing in the US. Fentanyl overdoses account for about two-thirds of all drug overdose deaths in the US. Here's more