R. S. Farag, M. A. El Nawawy, W. M. Fathy, and H. Atiaa, “Effect of oral injection of tramadol, sildenafil and combined of tramadol and sildenafil on serum testosterone level for Wistar breed male rats”, IJCT, vol. 11, no. 9, pp. 2959–2966, Dec. 2013.