Losartan is a common Angiotensin II Receptor Blocker (ARB) primarily used to treat high blood pressure (hypertension). It works by blocking the effects of angiotensin II, allowing blood vessels to relax and widen, which lowers blood pressure. This therapeutic action also makes it useful in managing heart failure and protecting the kidneys in people with type 2 diabetes. While Losartan is generally well-tolerated for long-term use, it carries a potential for adverse effects, including severe, uncommon reactions that require immediate medical attention.
Distinguishing Common Minor Effects from Serious Reactions
Most individuals starting Losartan experience relatively mild side effects, which are not considered the “worst” reactions and often diminish as the body adjusts to the drug. These frequent, non-serious symptoms can include a general feeling of tiredness or fatigue, along with mild dizziness. Upper respiratory symptoms are also commonly reported, such as a stuffy nose or congestion, and sometimes a minor cough. Other common, manageable effects may involve digestive upset, such as diarrhea, or musculoskeletal discomfort like back or joint pain. These issues are typically transient and rarely necessitate stopping the medication or seeking emergency care.
Acute, Life-Threatening Emergency Reactions
The most immediate and dangerous adverse effect associated with Losartan is angioedema, a severe, sudden allergic reaction. This involves rapid, localized swelling beneath the skin and mucous membranes, often affecting the face, lips, tongue, or throat. Swelling in the tongue or larynx is extremely dangerous because it can quickly compromise the airway, leading to difficulty breathing or swallowing. Although Losartan is an ARB and generally has a lower risk of angioedema than the related class of ACE inhibitors, the reaction is still possible. A generalized severe hypersensitivity reaction, or anaphylaxis, is also a rare but acute emergency, with symptoms including a widespread rash, intense itching, fever, or signs of circulatory collapse. Any symptom suggesting immediate airway compromise requires calling emergency services instantly.
Monitoring Systemic Risks and Organ Function
Hyperkalemia Risk
Losartan’s mechanism of action, which targets the renin-angiotensin system, can lead to severe, non-acute issues affecting long-term physiological balance, requiring consistent medical monitoring. A major concern is hyperkalemia, or abnormally high potassium levels in the blood. Losartan causes the body to retain potassium, and severe hyperkalemia is dangerous because it can disrupt the electrical signaling of the heart, leading to irregular heartbeats or even cardiac arrest. While mildly elevated potassium may be asymptomatic, more severe hyperkalemia can manifest as muscle weakness, fatigue, or a noticeable irregular heartbeat. Regular blood tests are necessary to check serum potassium levels, especially in patients with pre-existing conditions like diabetes or kidney impairment.
Kidney Function
Another significant systemic risk is the potential for acute kidney injury (AKI) or worsening of pre-existing renal disease. The drug can sometimes cause a temporary decrease in kidney function, especially when first started, due to changes in blood flow within the kidney. This is generally monitored by measuring markers like creatinine and blood urea nitrogen (BUN) in the blood. A sudden decrease in urine output or swelling in the extremities are possible signs of kidney impairment that warrant urgent physician review.
Critical Contraindications and High-Risk Drug Interactions
Pregnancy and Fetal Risk
The most absolute and serious contraindication for Losartan is its use during pregnancy, particularly during the second and third trimesters. The medication carries a Black Box Warning, the most serious warning from the FDA, due to the high risk of fetal toxicity. Exposure during these later stages of development can severely reduce the fetus’s renal function, leading to oligohydramnios (low amniotic fluid). This low fluid level can then cause fetal lung hypoplasia, skeletal deformation, and potentially fetal death. Losartan must be discontinued as soon as pregnancy is detected to avoid this significant risk.
Drug Interactions
Losartan also has several high-risk interactions that can amplify its most serious side effects. Combining it with potassium-sparing diuretics, such as spironolactone, or taking potassium supplements significantly increases the risk of severe hyperkalemia. This combination can dangerously overload the body’s potassium regulatory mechanisms. Another serious interaction involves the mood stabilizer Lithium. Losartan interferes with the kidney’s ability to clear lithium from the body, leading to a build-up that can cause lithium toxicity. Symptoms of this toxicity include confusion, muscle weakness, and tremors, necessitating careful monitoring of serum lithium levels when these medications are co-prescribed.