“Synthetic cannabis” is a common, but misleading, term that refers to a class of substances more accurately called cannabinoid receptor agonists or synthetic cannabinoids. Whereas cannabis usually refers to the dried flowered buds of the actual plant, which derives its main psychoactive effect through THC, synthetic cannabinoids get their name from their action on various cannabinoid receptors in the brain.
Sacrificing accuracy for simplicity, people in public office, the media, and law enforcement use the term “synthetic cannabis” or the brand names of products sold, such as “Spice” or “K2,” that are known to contain various synthetic cannabinoids. “Spice” is now widely used to mean any and all synthetic cannabinoids, rather than any individual compound.
Many of these substances have different chemical structures than THC, the main psychoactive ingredient in cannabis, and bind very much stronger (10-80 fold) than THC to receptors in the brain. This means that they can produce very different effects than THC and is one possible reason for the higher rates of some of the more extreme side effects that are only occasionally seen in adverse reactions to cannabis.