The basic problem is that marijuana is extremely tough to study. It is a veritable chemical warehouse containing over 400 chemical compounds in its smoke. Of these chemicals, over 50, called cannabinoids, are found only in marijuana. True, only one such chemical​—delta-9-THC—​is believed primarily responsible for the drug’s intoxicating effect. But since marijuana is grown under varying conditions, marijuana varies greatly in potency from batch to batch. This can wreak havoc with test results.a Further complicating matters is the fact that marijuana is generally inhaled, not injected. It is therefore very difficult to give test subjects a controlled dose, short of injecting them with delta-9-THC. Doing that, though, does not tell scientists how that chemical affects humans when it is inhaled along with over 400 companion chemicals.