How Does Losartan Work? Mechanism of Action Explained in Plain English

Losartan is a medication most people know for what it does — lowers blood pressure — but not necessarily for how it does it. Understanding the mechanism can help you understand why you're taking it, what side effects to expect, and why your doctor monitors certain things. Let's break it down in plain language.

The Renin-Angiotensin-Aldosterone System (RAAS): Your Body's Blood Pressure Control System

Your body has a built-in system to regulate blood pressure called the renin-angiotensin-aldosterone system, or RAAS. Here's how it works:

When your blood pressure drops or your kidneys sense low blood flow, they release an enzyme called renin.

Renin converts a protein in your blood into angiotensin I (inactive).

An enzyme called ACE (angiotensin-converting enzyme) then converts angiotensin I into angiotensin II (active and powerful).

Angiotensin II is a potent vasoconstrictor — it causes blood vessels to tighten and narrow, raising blood pressure. It also stimulates your adrenal glands to release aldosterone, a hormone that makes your kidneys retain sodium and water (also raising blood pressure).

In people with hypertension, this system is often overactive — blood vessels stay too tight, retaining too much fluid, keeping blood pressure chronically elevated.

How Losartan Blocks This System

Losartan works downstream in this chain. Instead of preventing angiotensin II from being made (like ACE inhibitors do), it blocks angiotensin II from binding to its receptor — specifically a receptor on blood vessels called the AT1 receptor.

Think of angiotensin II as a key, and the AT1 receptor as a lock. When the key fits the lock, blood vessels constrict. Losartan is like a fake key that fits the lock but doesn't turn it — blocking the real key from working.

Losartan is more than 10,000 times more selective for the AT1 receptor than any other angiotensin receptor. This selectivity helps it work effectively with a specific, targeted mechanism.

By blocking AT1, losartan prevents angiotensin II from:

• Constricting blood vessels (so blood pressure drops)
• Releasing aldosterone (so kidneys excrete more sodium and water, reducing fluid buildup)
• Causing cardiac remodeling (thickening of the heart muscle wall — relevant for LVH)
• Stimulating thirst, noradrenaline release, and other nervous system effects that raise blood pressure

The Active Metabolite: EXP3174

After you take losartan, about 14% of it is converted by your liver into an active metabolite called EXP3174. This metabolite is 10-40 times more potent at blocking AT1 receptors than losartan itself, and it stays in your system for 6-8 hours — contributing significantly to the drug's overall blood pressure-lowering effect throughout the day.

This is why people with poor CYP2C9 liver enzyme activity (a genetic variation) may have a weaker response to losartan — they can't convert it to EXP3174 as efficiently. In those cases, other ARBs that don't rely on this metabolic conversion may work better.

How Is Losartan Different From ACE Inhibitors?

ACE inhibitors (like lisinopril, enalapril, or ramipril) work one step earlier in the RAAS pathway — they block ACE and prevent angiotensin II from being formed at all. Losartan, by contrast, lets angiotensin II form but blocks it from doing anything at its receptor.

The clinical difference: ACE inhibitors block a different enzyme (ACE/kininase II) that also breaks down bradykinin. When bradykinin builds up, it irritates the throat — causing the persistent dry cough that affects 5-20% of ACE inhibitor users. Because losartan doesn't affect bradykinin at all, it doesn't cause this side effect.

Why Does Losartan Protect the Kidneys?

In diabetes, high blood sugar damages the small blood vessels in the kidneys. Angiotensin II worsens this damage by increasing pressure within the kidney's filtering units (glomeruli) and promoting scarring. By blocking AT1 receptors in the kidney, losartan reduces this internal pressure, slows protein leakage into the urine, and preserves kidney function over time.

This kidney-protective effect is independent of — and additive to — losartan's blood pressure-lowering effects. It's one reason ARBs and ACE inhibitors are preferred blood pressure medications specifically for patients with diabetes or chronic kidney disease.

For the full patient guide on uses and dosage, see: What Is Losartan? Uses, Dosage, and What You Need to Know in 2026

Learn about drug interactions: Losartan Drug Interactions: What to Avoid.