The scientist behind synthetic cannabis says the products "absolutely should not be used as recreational drugs".
Emeritus Professor John W Huffman, of South Carolina, developed synthetic cannabinoid compounds that bear his initials (such as JWH-018 and JWH-073) in his laboratory more than 15 years ago.
Those compounds were widely discussed in scientific publications, and have now been used to make recreational drugs.
He said the compounds were "not meant for human consumption".
"Their effects in humans have not been studied and they could very well have toxic effects.