This paper reviews the role of cannabis in diabetes. Cannabis is by far the most commonly used illicit drug in Britain, though its use may be declining. There are an estimated 50,000–100,000 people with diabetes using cannabis, with an unknown number using the drug for self-medication. The evidence of the effects of cannabis on diabetes is complex, ranging from anecdotal reports of benefits and harms to experimental research on cannabinoids. The endocannabinoid system appears to have a role in the regulation of body weight and food intake, and the development of hyperglycaemia, insulin resistance and dyslipidaemia. In experimental models, the main psychoactive constituent of herbal cannabis, Δ9-tetrahydrocannabinol, has been shown to interfere with both the action of insulin and its release. The paper also considers the effects of cannabis on complications of diabetes. Experimental work has suggested a mechanism to reduce neuropathy but the only double-blind clinical trial to date of a cannabis-based drug found no difference in the ability of the cannabis-based product to relieve neuropathic pain when compared with placebo. In conclusion, new insights into the role of cannabis and cannabinoids in diabetes are emerging from this developing field of research.