Objectives Whether and how synthetic cannabinoids affect inflammation and carcinogenesis has not been well studied. The present study was thus conducted to assess effects of synthetic cannabinoids on inflammation and carcinogenesis in vivo in mice.

Methods Twenty‐three analogues of synthetic cannabinoids were isolated from, and identified as adulterants in, illegal drugs distributed in the Tokyo metropolitan area, and were examined for their inhibitory effects on the induction of oedema in mouse ears by 12‐O‐tetradecanoylphorbol‐13‐acetate (TPA). Furthermore, selected cannabinoids, JWH‐018, ‐122 and ‐210, were studied for their effects on carcinogenesis induced in mouse skin initiated with 7,12‐dimethylbenz[a]anthracene (DMBA) and promoted by TPA.

Key findings Among cannabinoids, naphthoylindoles mostly exhibited superior inhibitory effects against TPA‐induced ear oedema and, especially, JWH‐018, ‐122 and ‐210 showed potent activity with 50% inhibitory dose (ID50) values of 168, 346 and 542 nm, respectively (an activity corresponding to that of indometacin (ID50 = 908 nm)). Furthermore these three compounds also markedly suppressed the tumour‐promoting activity of TPA.