What Cialis is

Tadalafil is a phosphodiesterase type 5 inhibitor. PDE5 breaks down cyclic GMP, and inhibiting it allows cGMP to accumulate where nitric oxide signalling has already begun, which is why the class requires that signalling rather than initiating it. What separates tadalafil from the rest of the class is duration: its half life is roughly 17.5 hours against about four for sildenafil, an order-of-magnitude practical difference that comes from structure rather than dose. It is also structurally unrelated to sildenafil and vardenafil, which are close analogues of each other; tadalafil is a different chemotype entirely.

How Cialis compares

Against sildenafil: same enzyme target, very different duration, and different chemical structure. Sildenafil's roughly four hour half life against tadalafil's 17.5 is the single most consequential difference in the class. Against vardenafil: closer to sildenafil than to tadalafil in both structure and duration.