Impotence or erectile dysfunction is a condition in men when they cannot achieve or sustain an erection strong enough to engage in sexual intercourse. It is caused by insufficient blood flow to the penile area. Filagra is the brand name of Sildenafil, which helps men get and maintain an erection by increasing blood flow to the penis. The effect of this medicine lasts for 4 to 6 hours. You can take this medicine before or after food.