Mostafa, T., L. A. Rashed, D. A. Sabry, I. Osman, N. Nabil, F. Kareem, and I. A. Mostafa,  "Serum L-carnitine and vitamin D levels may be low among oral sildenafil citrate non-responders.",  International journal of impotence research, vol. 31, issue 2, pp. 85-91, 2019 Mar.