If you have high blood pressure, you're far from alone. Roughly one-third of adults in the United States have high blood pressure, according to the Centers for Disease Control and Prevention. Losartan (Cozaar) belongs to a class of drugs called the angiotensin receptor blockers, or ARBs. It is recommended as a first-choice option to treat high blood pressure by the 2014 Joint National Committee guidelines. By blocking angiotensin receptors in the blood vessels and other locations in the body, losartan leads to effects that reduce blood pressure. These effects include prevention of blood vessel constriction, reduced retention of water by the kidneys and decreased sympathetic nervous system activity.

Blood Vessel Effects

Losartan acts by blocking the angiotensin receptors. Angiotensin is a substance produced by the body to maintain blood pressure when it binds to its receptors in body tissues. When angiotensin binds to receptors found in blood vessels, it causes the vessels to constrict. This causes an increase in blood pressure. Under normal circumstances, this mechanism ensures normal blood flow. In people with high blood pressure, this mechanism provides an opportunity to decrease blood pressure. When losartan binds to angiotensin receptors, it leaves no space for angiotensin to bind. In this way, losartan keeps blood vessels relaxed, and blood pressure does not increase.

Aldosterone Effects

Losartan also blocks the angiotensin receptors in the adrenal glands, which sit on top of the kidneys. The adrenals produce several hormones that serve various functions in the body. Losartan blocks the ability of angiotensin to cause adrenal release of a hormone called aldosterone. Aldosterone stimulates the kidneys to increase the water and sodium content in the body. The added water and sodium expand the blood volume, which increases blood pressure. By blocking the angiotensin receptors in the adrenal glands, losartan interferes with this mechanism of increasing the blood pressure .

Sympathetic Nervous System Effects

Blood pressure is affected by a branch of the nervous system called the sympathetic nervous system. When messages from this branch of the nervous system increase, blood pressure increases. This occurs because the increased sympathetic nervous system activity causes the heart to pump more blood and the blood vessels to constrict. Angiotensin increases the activity of the sympathetic nervous system. By inhibiting the activity of angiotensin, losartan blocks increased sympathetic nervous system input into the heart and blood vessels, which prevents an increase in blood pressure related to this mechanism.

Warnings and Precautions

According to the U.S. Food and Drug Administration-approved prescribing information, losartan is generally well-tolerated, but side effects are possible. The most common side effects include fatigue, muscle aches, dizziness and diarrhea. Serious but rare side effects include allergic reactions, swelling of the lips or throat, liver failure, kidney failure, and a drop in the red and/or white blood cells. Talk with your doctor if you experience new symptoms while taking losartan. However, it's important not to stop taking your medicine without your doctor's approval.

Losartan can injure the developing baby during pregnancy, especially during the fourth to ninth month. Seek immediate medical attention if you experience swelling of the tongue, face or lips -- especially if you have difficulty breathing.