Cannabis sativa preparations are the most widely used illicit drugs worldwide. Silymarin or milk thistle is a widely used hepatoprotective agent. The present study examined the possible modulation by Cannabis sativa preparation of the hepatoprotective effect of silymarin. Mice were pretreated with Cannabis sativa extract (10 or 20 mg/kg; expressed as Δ⁹-tetrahydrocannabinol), silymarin (25 mg/kg) or cannabis combined with silymarin for 5 days prior to the administration of carbon tetrachloride (CCl₄) and mice were euthanized 24 h post-CCl₄. Alternatively, cannabis alone (20 mg/kg) or cannabis combined with silymarin (25 mg/kg) was given simultaneously with CCl₄ and mice were euthanized 5 days later. Liver damage was assessed by determining hepatic enzyme activities and hepatic histopathology. Malondialdehyde (MDA), reduced glutathione (GSH), nitric oxide (the concentrations of nitrite/nitrate) and paraoxonase (PON 1) activity were determined in the liver. Results indicated that mice pretreated with silymarin showed significantly increased GSH, decreased MDA, nitric oxide as well as liver transaminases. Cannabis sativa given prior to CCl₄ resulted in increased liver nitric oxide and transaminases, but failed to alter GSH or PON1 activity. In contrast, cannabis given simultaneously with CCl₄ resulted in increased MDA, nitric oxide, transaminases and decreased GSH and PON1 activity compared with the silymarin + cannabis treatment group. The histopathological studies confirmed these findings. The present results thus suggest decreased effectiveness of silymarin by cannabis administration.