What are PDE5 inhibitors for treatment of erectile dysfunction (ED)?

PDE5 inhibitors -- sildenafil (Viagra), tadalafil (Cialis), vardenafil (Levitra) -- are pills used to treat erectile dysfunction (ED). They increase blood flow to the penis by augmenting cyclic guanosine monophosphate (GMP), a chemical that relaxes muscles in the penis, improving blood flow during sexual stimulation.

All three drugs have similar effectiveness (about 60% to 70%), but tadalafil lasts about 36 hours, whereas sildenafil and vardenafil last about eight hours.