How does sildenafil actually work?

Sildenafil is a PDE5 inhibitor. It blocks the enzyme PDE5, which normally breaks down cGMP. By maintaining high cGMP levels, smooth muscle in the blood vessels relaxes, allowing massive increases in regional blood flow.

Why is tadalafil called the weekend pill?

Tadalafil has a half-life of 17.5 hours, meaning it remains active in the body for up to 36 hours. Sildenafil has a half-life of 4 hours.

Guide

Sildenafil vs Tadalafil: The Clinical Differences Explained

A deep dive into how PDE5 inhibitors work to increase blood flow, and the medical differences between daily and on-demand protocols.

Updated April 2026Editorially ReviewedEditorial Independence

The PDE5 Inhibition Mechanism

To understand the difference between ED medications, you have to understand the biological mechanism. When arousal occurs, the body releases nitric oxide (NO), which stimulates the production of cGMP. This molecule signals the smooth muscles in the blood vessels to relax, allowing blood to flow in.

However, the body naturally produces an enzyme called PDE5, which breaks down cGMP. If PDE5 breaks down cGMP too quickly, the vessels constrict again.

Mechanism of Action: PDE5 Inhibitors

How Sildenafil and Tadalafil improve blood flow

Constricted Vessel

Enzyme PDE5 naturally breaks down cGMP, keeping vascular smooth muscle constricted.

Medication Taken

PDE5 Inhibition

Sildenafil/Tadalafil blocks PDE5. cGMP levels rise rapidly during sexual arousal.

Vasodilation

Smooth muscle heavily relaxes, expanding the vessel and dramatically increasing blood flow.

Sildenafil (Viagra) and Tadalafil (Cialis) are both 'PDE5 Inhibitors.' They temporarily block this enzyme, allowing cGMP levels to stay high enough to maintain vasodilation.

Half-Life and Duration

The most fundamental difference between the two most popular ED drugs is their pharmacokinetics—specifically, their half-life.

**Sildenafil** has a half-life of roughly 4 hours. It is designed to be taken on an 'as-needed' basis (PRN) about 30-60 minutes before sexual activity. It enters the system quickly and leaves quickly.

**Tadalafil** has a half-life of 17.5 hours. A single dose can remain effective for up to 36 hours. Because of this long pharmacological tail, tadalafil can be taken on a daily basis at a low dose (typically 2.5mg or 5mg), which maintains steady-state levels of the drug in the bloodstream.