As Dr. Campbell suggests, these findings should not be "unduly extended" and, of course, need to be replicated by independent laboratory experiments, but our research does provide preclinical evidence of what has long been clear from clinical experience, that marijuana has abuse potential like other abused drugs and is a potential medical problem. The principal guide for the medical community in diagnosing psychiatric disorders is the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-4, 2000) published by the American Psychiatric Association. It clearly describes marijuana (cannabis) as an abused drug capable of producing dependence and intoxication (pages 236 to 242). Marijuana is indeed illegal in the United States, Great Britain and much of the rest of the world. When I am writing about caffeine, I do refer to it being the most used psychoactive agent in the world, but marijuana is not caffeine and it is illegal.
Finally, the presence of a "ceiling effect" as the injection dose of THC was increased in our self-administration studies is exactly the same effect seen with other abused drugs such as cocaine or heroin under the same conditions. It is commonly referred to as an inverted u-shaped dose-response curve. However, the presence of an inverted u-shaped dose-response curve under controlled experimental conditions does not mean that a drug such as cocaine or heroin (or THC in marijuana) is free of toxicity and safe for human consumption.