Fildena 100 is a medication used to treat erectile dysfunction (ED). It contains sildenafil citrate, which is the same active ingredient found in Viagra. Sildenafil works by increasing blood flow to the penis, helping to achieve and maintain an erection in response to sexual stimulation. It's typically taken about 30 to 60 minutes before sexual activity. The effects can last for up to four hours, but it doesn't cause an erection without sexual arousal. Common side effects can include headaches, flushing, nasal congestion, and digestive issues. More serious side effects, though rare, can include vision changes or priapism (a prolonged and painful erection).