Cialis works by supporting the body’s natural erection process, not by forcing an erection. This page explains how Cialis works inside the body, why sexual stimulation is required, and why tadalafil’s effects last longer than many other ED medicines.

For the complete overview, see the guide.

Table of Contents

The Normal Erection Process (Quick Context)

An erection begins with sexual stimulation (physical or psychological), which triggers nerve signals in the penis. These signals cause the release of nitric oxide (NO), a chemical messenger that relaxes smooth muscle in penile blood vessels. As vessels relax, blood flow increases, leading to an erection.

If this pathway is disrupted-by vascular disease, diabetes, nerve damage, or ageing-erections become weak or unreliable.

What the PDE5 Enzyme Does

Inside penile tissue, an enzyme called phosphodiesterase type-5 (PDE5) breaks down a messenger molecule known as cGMP.
cGMP is responsible for keeping blood vessels relaxed during an erection.

High PDE5 activity → cGMP breaks down too quickly
Result → blood vessels tighten sooner → erection fades or fails

How Cialis Changes This Pathway

Cialis (tadalafil) is a PDE5 inhibitor. It works by:

Blocking the PDE5 enzyme
Allowing cGMP to remain active for longer
Keeping blood vessels relaxed
Supporting sustained blood flow during arousal

The result is a stronger and more reliable erectile response-but only when sexual stimulation occurs.

This is why Cialis does not cause automatic erections and will not work without arousal.

Why Sexual Stimulation Is Required

Cialis does not release nitric oxide on its own.
Sexual stimulation is still needed to start the NO → cGMP pathway.

Without stimulation:

NO is not released
cGMP is not produced
Cialis has nothing to “support”

This is a frequent cause of “Cialis didn’t work” experiences, explored further in (What to Do If Cialis Doesn’t Work).

Why Cialis Lasts Longer Than Other ED Medicines

Tadalafil has a longer half-life than many other PDE5 inhibitors. This means:

It stays active in the bloodstream for longer
PDE5 inhibition is sustained over many hours
The erectile response window is extended

This pharmacological property is why Cialis can:

Work up to 36 hours in many users
Be taken as a daily low-dose option

Timing and duration are explained in detail in (How Long Cialis Lasts).

Why Blood Pressure Can Drop

Because Cialis relaxes blood vessels, it can cause a mild drop in blood pressure.
For most healthy users this is not dangerous, but it becomes risky when combined with:

Nitrate medicines
Certain alpha-blockers
Excessive alcohol

This explains key safety warnings covered in (Who Should Not Take Cialis) and (Drug Interactions).

Cialis and the Prostate (BPH Mechanism)

When taken daily, tadalafil also relaxes smooth muscle in:

The prostate
The bladder neck

This reduces resistance to urine flow, helping relieve BPH urinary symptoms-a secondary but clinically important effect discussed in (Cialis for BPH).

What Cialis Does NOT Affect

To avoid confusion:

It does not raise testosterone
It does not alter sperm production
It does not directly affect libido
It does not permanently “fix” ED

It supports physiology; it does not replace it.

Key Takeaways

Cialis works by inhibiting PDE5, preserving cGMP
It supports erections only with sexual stimulation
Longer half-life explains long duration and daily dosing
Blood-vessel relaxation explains both benefits and risks
The same mechanism helps BPH symptoms when taken daily