Sildenafil is a medicine used to treat erectile dysfunction, it is the generic name for Viagra, It works by increasing the blood flow to the penis to help men get an erection.  Studies have shown that at least two-thirds of men have improved erections after taking it. Sildenafil can be taken up to 4 hours before sexual intercourse, it usually takes 30-60 minutes for the medicine to achieve maximum concentration , sexual arousal is required for the medicine to work.