Protective effect of Nigella sativa oil against tramadol-induced tolerance and dependence in mice: Role of nitric oxide and oxidative stress

► Nigella sativa oil inhibited the development of tramadol tolerance and dependence. ► N. sativa oil inhibited tramadol-induced NO overproduction and oxidative stress. ► N. sativa oil maintained the intracellular antioxidant status. ► The oil appears to be an alternative to treat tramadol tolerance and dependence.