I used to be an organic chemist. These fake marijuana substitutes are pretty horrific: unscrupulous vendors read the scientific literature for newly developed cannabinoid receptor activating chemicals. These are then sold, without proper testing in humans usually.

The side effect profile of these compounds is worse than natural cannabis, with high incidence of psychotic symptoms.

https://en.m.wikipedia.org/wiki/Synthetic_cannabinoids

Edit: i completely overlooked rpedroso's substantially identical reply, haha...