Serum l carnitine and vitamin D levels may be low among oral sildenafil citrate non responders International Journal of Impotence Research Understanding Loss of Sensitivity in Penis: Causes and Evidence Based Approaches Acetyl l carnitine plus propionyl l carnitine improve efficacy of sildenafil in treatment of erectile dysfunction after bilateral nerve sparing radical retropubic prostatectomy ScienceDirect Acetyl L Carnitine and Erectile Dysfunction