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The scientist behind synthetic cannabis says the products ''absolutely should not be used as recreational drugs''.
Emeritus Prof 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 ''evidently some people have figured out how to make them and are putting them in products''.
The compounds were ''not meant for human consumption'', he said.
''Their effects in humans have not been studied and they could very well have toxic effects. They absolutely should not be used as recreational drugs.''
Prof Huffman, an organic chemist at Clemson University, said the compounds which are appearing in ''fake marijuana'' products were among many created by his group, and others, during research supported by the National Institute on Drug Abuse between 1984 and 2010.
That research aimed to understand the relationship between the chemical structure and the biological activity of substances known as cannabinoids.
Cannabinoids include THC - the active ingredient in cannabis plants - but also other substances that interact with the cannabinoid receptors in the brain and other organs, he said.
''These receptors don't exist so that people can smoke marijuana and get high; they play a role in regulating appetite, nausea, mood, pain and inflammation.''
The receptors could be involved in the development of conditions such as osteoporosis, liver disease and some kinds of cancer, Prof Huffman said.
''Synthetic cannabinoids can help us understand these interactions and, ultimately, this knowledge may contribute to the development of new therapies.
''Again, I would emphasise the risk people are taking when they smoke these products. We simply don't know what the health effects might be.''