Cialis works by inhibiting the enzyme PDE5. This enzyme keeps the cGMP down which contrasts the blood vessels and slowdown blood flow towards the sexual organ of the male. Cialis works by inhibiting the enzyme PDE5. This enzyme keeps the cGMP down which contrasts the blood vessels and slowdown blood flow towards the sexual organ of the male. When Cialis drug is swallowed by water by male, Tadalafil 20mg, the chemical in it prevents PDE5 from working. This allows nitrate oxide to stimulate cGMP for relaxing blood vessels. Relaxed blood vessels facilitate blood flow in the body. The blood flow in the blood vessels of the penis makes it easy to get an erection. Any dose of the Cialis has the same function.