Losartan

*Losartanum*

For medical students2 min readUpdated 2026-10-10

Losartan is an antihypertensive drug that is an imidazole derivative in its chemical structure. It blocks AT_1 receptors, effectively reducing vascular tone and blood pressure, and is also characterized by a specific ability to promote the excretion of uric acid from the body.

Chemical structureImidazole derivative

MetabolismConverted in the liver into a metabolite 10–40 times more active

Half-lifeParent drug: 1.5–2 hours; active metabolite: 6–9 hours

Key featureExhibits pronounced uricosuric action

Chemical Nature and Pharmacokinetic Features

In terms of its chemical structure, losartan is an imidazole derivative. When administered orally, the drug is absorbed quite rapidly from the gastrointestinal tract. However, its bioavailability is relatively low, at only 30–35%. This means that only about a third of the administered dose reaches the systemic circulation unchanged.

A key pharmacokinetic feature of the drug is its intensive metabolism. Passing through the liver, the parent substance undergoes biotransformation, resulting in the synthesis of an active metabolite. This metabolite plays a decisive role in the therapeutic effect, as its pharmacological activity exceeds that of losartan itself by 10–40 times.

The difference between the parent compound and its derivative is also evident in their elimination rates. The half-life ($t_{1/2}$) for losartan itself is very short, ranging from 1.5 to 2 hours. At the same time, the active metabolite circulates in the blood significantly longer, with a half-life reaching 6–9 hours. Due to this prolonged presence of the active component in the blood, patients need to take the drug only once daily to maintain a stable effect.

Pharmacodynamics and Dual Antagonism

The mechanism of action is based on the blockade of AT_1 receptors; however, this process is dual in nature due to the presence of two active molecules in the patient's blood:

Losartan itself acts as a competitive antagonist, competing with endogenous ligands for receptor binding.
The active hepatic metabolite acts as a non-competitive antagonist of AT_1 receptors, binding in a way that ensures a more potent and prolonged blockade.

Systemic Pharmacological Effects

Receptor blockade triggers a cascade of reactions that ultimately normalize the patient's condition. The main effects can be divided into several categories:

Hemodynamic impact: The drug effectively reduces vascular tone. This naturally leads to a decrease in total peripheral resistance (TPR) and a sustained drop in blood pressure. In addition, vasodilation significantly reduces cardiac afterload, facilitating myocardial function.
Water-electrolyte balance: The drug decreases aldosterone secretion. Lowering this hormone triggers natriuretic and diuretic effects, causing the body to excrete sodium ions and excess fluid more actively.
Specific feature: Unlike many other antihypertensive drugs, losartan possesses a pronounced uricosuric effect, meaning it specifically promotes the excretion of uric acid from the body.

Safety Profile and Side Effects

Despite its high efficacy, therapy may be accompanied by adverse reactions. The side effects of losartan are largely similar to the general adverse profile of the entire drug class.

Among the most frequent complaints are systemic dizziness and fatigue. Physicians and patients must also be aware of the risk of allergic reactions, particularly angioedema.

Mnemonic

To remember the drug's profile, use the association: "LOSARTAN is a vine (loza) that roots in the liver and drops acidic leaves." "Roots in the liver" reminds you that the active metabolite (10–40 times stronger than the parent drug) is formed there. "Drops acidic leaves" refers to its unique uricosuric action (excretion of uric acid). Also, remember the double play: the parent substance is a competitive receptor competitor, while its metabolite is a non-competitive one.

Frequently asked questions

What are the main indications and contraindications for prescribing losartan in clinical practice?

Main evidence-based clinical applications of losartan:

Indications / Clinical situations:

Hypertension: Losartan belongs to angiotensin II receptor blockers (ARBs), which are among the core classes of antihypertensive medications; ARBs can be used as initial therapy in combination with a calcium channel blocker or diuretic.
Heart failure: Losartan is an ARB used in heart failure; it serves as an alternative when ACE inhibitors are not tolerated, often due to dry cough. ARBs, including losartan, are also listed as alternatives when ARNI or ACE inhibitor intolerance occurs.
Stable ischemic heart disease with comorbid conditions affecting prognosis: ARBs are prescribed for the same indications as ACE inhibitors in cases of ACE inhibitor intolerance; losartan is listed among the ARB drugs of choice for ischemic heart disease and heart failure.
IgA nephropathy: Dosages for losartan include a mean study dose of 0.8 mg/kg/day, and a starting dose for children aged 6–9 years of 0.7 mg/kg once daily.

Contraindications:

Explicit contraindications are not detailed in the source text.

Side effects / limitations noted in sources:

For ARBs: hyperkalemia, elevated creatinine, teratogenicity; cough and angioedema occur less frequently than with ACE inhibitors.
For losartan: adverse effects are similar to class-wide reactions, including dizziness, fatigue, and angioedema.

What is the chemical nature of losartan?

Chemically, the drug is an imidazole derivative.

Why is losartan prescribed only once a day despite its short half-life?

Losartan itself is eliminated quickly (half-life of 1.5–2 hours). However, the liver metabolizes it into an active metabolite that is 10–40 times more potent than the parent compound. The half-life of this metabolite is 6–9 hours, providing a sustained therapeutic effect throughout the 24-hour period.

How does the drug lower blood pressure?

Blood pressure is reduced by decreasing vascular tone and total peripheral resistance (TPR). An additional contribution comes from reduced aldosterone secretion, leading to the excretion of sodium and excess fluid.

What is the main specific difference between losartan and other drugs?

Its unique feature is its uricosuric action. The drug not only lowers blood pressure but also actively promotes the excretion of uric acid from the body.

Go deeper

Biotransformation of imidazole derivatives: how the liver synthesizes active metabolites
Hemodynamics in detail: the relationship between lowered TPR and cardiac afterload
Receptor interaction mechanisms: competitive and non-competitive antagonism of AT1 receptors
The role of aldosterone in water-electrolyte balance and mechanisms of natriuresis
Pathophysiology of angioedema as a class-wide adverse effect