Combination therapy with topical alprostadil and phosphodiesterase 5 inhibitors after failure of oral therapy in patients with erectile dysfunction: a prospective, two arm, open label, non randomized study International Journal of Impotence Research L Carnitine Erectile Dysfunction: What the Evidence Actually Shows Serum l carnitine and vitamin D levels may be low among oral sildenafil citrate non responders International Journal of Impotence Research C3) Propionyl L carnitine (N methyl d3) CAS#:1334532 19 2