This assessment of marijuana's abuse potential is from the following article: Felder, Christian, and Michelle Glass. 1998. Cannabinoid receptors and their endogenous agonists. Annual Review of Pharmacology and toxicology, 38:179-200:

Much of the political and public objection to the use of Delta-9- THC or marijuana as a therapy centers around its abuse potential and the belief by some that it serves as a "gateway" drug leading users to "harder" drugs of abuse. Many therapeutic drugs have abuse potential, yet this does not invalidate their role in current therapies. While there is some preliminary evidence for cannabinoids activating the reward pathways in the brain (Tanda et al. 1997), most investigators have failed to find addictive or reinforcing effects of cannabinoids in animal models. Unlike cocaine or heroin, cannabinoid agonists produce conditioned place aversion even at low doses (McGregor et al. 1996; Parker and Gilles 1995) and anxiogenic effects (Onavi et al 1990). Furthermore, animals will not self-administer cannabinoids (Harris et al 1974; Leite and Carlina 1974; Cocoran and Amit 1974), and a lack of cross-sensitization between cocaine (McGregor et al 1995) or amphetamines (Takahashi and Singer 1981) and cannabinoids has also been demonstrated.

Sources cited by Felder and Glass:

Cocoran, M.E.and Z. Amit. 1974. Reluctance of rats to drink hashish suspensions: free choice and forced consumption, and the effects of hypothalamic stimulation. Psychopharmacologia 35:129-47. Cited in Felder and Glass 1998.

Harris, R.T., W. Waters and D. McLendon. 1974. Evaluation of reinforcing capability of D⁹-tetrahydrocannabinol in rhesus monkeys. Psychopharmacologia 37:23-29. Cited in Felder & Glass, 1998.

Leite, J.R. and E.A. Carlina. 1974. Failure to obtain cannabis-directed behavior and abstinence syndrome in rats chronically treated with cannabis sativa extracts. Psychopharmacologia 36:133-45. Cited in Felder and Glass 1998.

McGregor, I.S., P.A. Bryant, and J. Arnold. 1995. CP55,940, a synthetic cannabinoid, does not sensitize locomotor activity or cocaine responsitivity with intermittent administration in Wistar rats. Soc. Neurosci. Abstr. 21:726. Cited in Felder and Glass 1998.

McGregor, I.S., C.N. Issakidis, and G. Prior. 1996. Adverse effects of the synthetic cannabinoid CP55,940 in rats. Pharmacol. Biochem. Behav. 53:657-64. Cited in Felder and Glass 1998.

Parker, L.A. and T. Gilles. 1995. THC-induced place and taste aversion in Lewis and Sprague-Dawley rats. Behav. Neurosci. 109:71-78. Cited in Felder and Glass 1998.

Onavi, E.S., M.R. Green, and B.R. Martin. 1990. Pharmacological characterization of cannabinoids in the elevated plus maze. J Pharmacol. Exp. Ther. 253:1002-9. Cited in Felder and Glass 1998.

Takahashi, R.N. and G. Singer. 1981. Cross self-administration of ⁹-tetrahydrocannabinol and d-amphetamine in rats. Braz. J. Med. Biol. Res. 14:395-400. 1981. Cited in Felder and Glass 1998.

Tanda, G., F.E. Pontieri and G. Di Chiara. 1997. Cannabinoid and heroin activation of mesolimbic dopamine transmission by a common opiod receptor mechanism. Science. 276: 2048 - 2050. Cited in Califiano, 1998. Cited in Felder and Glass 1998.