Adolescent rats repeatedly injected with tetrahydrocannabinol increased the self-administration of heroin (results based on 11 male rats and >50 male rats), morphine (study based on 12 male rats) and also nicotine (34 rats). There were direct indications that the alteration consisted of lasting anatomical changes in the reward system of the brain. Because the reward system is anatomically, physiologically, and functionally almost identical across the class of mammals, the importance of the findings from animal studies for the reward system in the human brain in relation to the liability to the use of further drugs has been pointed out in several reviews.