Filagra Dosage is one of the most well-known drugs in the world for treating erectile dysfunction, or ED, or impotence. Sildenafil is the active ingredient in Filagra. By relaxing the blood vessels and penile muscles, it improves blood flow and makes it easier for men to achieve an erection. Take Filagra minutes after having sex in order to help with erection. Be advised that temporary vision changes and flushing are common side effects.