Fildena is a generic version of Viagra, and it contains the same active ingredient, sildenafil citrate. Fildena is used to treat erectile dysfunction. It works by relaxing muscles in the penis, which allows for better blood flow and helps to achieve and maintain an erection. The most common side effects of Fildena include headache, flushing, upset stomach, and back pain. Serious side effects are rare, but they can occur.