What are the side effects of taking cialis/viagra/levitra?

Constricts blood ves: Constricts blood vessels so the blood doesn't drain out of the penis, keeping the penis erect. Rare side effects such as back ache, head ache, loss of hearing or vision (rare), blue vision, very rare allergic reaction. Generally pretty safe, but can interact with some medication such as Alpha blockers for blood pressure a/o prostate enlargement.