Is Losartan Safe During Pregnancy? What Every Woman Should Know

TL;DR

• Losartan is contraindicated in all trimesters of pregnancy and carries an FDA Pregnancy Category X designation (formerly Category D in second/third trimesters, now explicitly labeled under the PLLR system as a drug that can cause fetal harm) [VERIFY].
• Exposure during the second and third trimesters is associated with potentially fatal fetal renal failure, oligohydramnios, skull hypoplasia, and neonatal death [VERIFY].
• Women of childbearing age taking losartan should use reliable contraception and switch to a pregnancy-compatible antihypertensive (e.g., labetalol, nifedipine, or methyldopa) as soon as pregnancy is confirmed [VERIFY].

§01What Is Losartan and Why Is Pregnancy a Concern?

Losartan (Cozaar) is the first orally available angiotensin II type 1 (AT₁) receptor blocker (ARB) to reach clinical practice [2][5]. Following oral administration, losartan is rapidly absorbed and partially converted — approximately 14% of the dose — to the active metabolite E-3174, which is 10- to 40-fold more potent than the parent compound and has an estimated terminal half-life of 6 to 9 hours [2]. The drug is widely prescribed for hypertension, diabetic nephropathy with proteinuria in type 2 diabetes [7], and, within the broader ARB class, chronic heart failure [1].

Losartan works by selectively blocking the AT₁ receptor, preventing angiotensin II from exerting its vasoconstrictive and aldosterone-stimulating effects. This mechanism — essential for blood pressure control in adults — becomes profoundly dangerous in a developing fetus. The renin-angiotensin-aldosterone system (RAAS) plays a critical role in fetal renal development, cardiovascular maturation, and amniotic fluid regulation. Blocking this system with losartan or any other ARB can disrupt fetal organ formation, particularly during the second and third trimesters when the kidneys are actively developing [VERIFY].

The pharmacokinetics review by Sica and colleagues (2005) states plainly: "Losartan should be avoided in pregnancy, as is the case with all other angiotensin-receptor antagonists" [2]. This warning applies regardless of the clinical indication for which the drug was originally prescribed, whether hypertension, nephroprotection, or heart failure.

§02How Losartan Harms the Developing Fetus

The fetal risks associated with losartan and other drugs that act on the RAAS (including ACE inhibitors such as enalapril, lisinopril, and ramipril) are well documented through decades of post-marketing surveillance, case reports, and animal studies. Although controlled clinical trials in pregnant women are — for obvious ethical reasons — not performed, the evidence from inadvertent exposures and animal reproductive toxicology is consistent and alarming [VERIFY].

Second- and third-trimester exposure

The most severe consequences occur when losartan is taken during the second or third trimester. During this period, fetal urine output is a major contributor to amniotic fluid volume, and the fetal kidneys depend on an intact RAAS for normal perfusion and development. Blocking the AT₁ receptor with losartan can cause:

• Oligohydramnios — reduced amniotic fluid volume resulting from decreased fetal urine production. Oligohydramnios in turn leads to limb contractures, pulmonary hypoplasia, and craniofacial deformities [VERIFY].
• Fetal renal failure — impaired renal perfusion can result in irreversible renal tubular dysgenesis. Neonates born after prolonged exposure may present with anuria requiring dialysis or ultimately incompatible with survival [VERIFY].
• Skull hypoplasia — reduced ossification of the fetal skull, sometimes referred to as calvarial hypoplasia, is a recognized feature of fetal ARB/ACE inhibitor toxicity [VERIFY].
• Neonatal hypotension — newborns exposed in utero may exhibit severe, refractory low blood pressure at birth [VERIFY].
• Neonatal death — in the most severe cases, the combination of renal failure, pulmonary hypoplasia, and cardiovascular instability proves fatal [VERIFY].

The FDA, ACOG (American College of Obstetricians and Gynecologists), and the European Medicines Agency (EMA) have all issued warnings that ARBs, including losartan, must be discontinued as soon as pregnancy is detected in the second or third trimester [VERIFY].

First-trimester exposure

The risk during the first trimester has been a subject of ongoing debate. Earlier data suggested that first-trimester exposure to RAAS inhibitors might be associated with an increased risk of cardiovascular and central nervous system malformations. However, subsequent studies have raised questions about confounding by the underlying hypertensive disorder itself. The current consensus, reflected in ACOG Practice Bulletin No. 203 (2019) and reinforced by the FDA's prescribing information updates, is as follows [VERIFY]:

• ARBs including losartan should be discontinued as soon as pregnancy is recognized, even in the first trimester.
• If first-trimester exposure has already occurred, the patient should be counseled about potential risks and offered detailed anatomical ultrasonography.
• The absolute risk of major malformations from first-trimester-only exposure appears lower than that from continued second/third-trimester use, but it is not considered negligible.

In practical terms, this means losartan is contraindicated throughout pregnancy, and the safest approach is to transition to a pregnancy-compatible drug before conception when possible.

§03Losartan vs. Other Antihypertensives in Pregnancy: A Safety Comparison

Note: The FDA replaced letter categories (A, B, C, D, X) with the Pregnancy and Lactation Labeling Rule (PLLR) in 2015. Legacy categories are shown here for reference because many clinicians still use them. Under the PLLR, losartan's label states it can cause fetal harm and should be discontinued when pregnancy is detected [VERIFY].

The contrast is striking. While losartan and the entire ARB/ACE inhibitor class carry the strongest possible warnings against use in pregnancy, multiple alternatives — labetalol, nifedipine, and methyldopa — have decades of safety data supporting their use during gestation. ACOG and NICE (National Institute for Health and Care Excellence, UK) guidelines both recommend labetalol as a first-line agent for chronic hypertension in pregnancy, with nifedipine as a suitable alternative [VERIFY].

§04What to Do if You Become Pregnant While Taking Losartan

Discovering a pregnancy while on losartan is not uncommon, particularly because about half of pregnancies are unplanned. The following steps are recommended by major clinical guidelines:

1. Do not panic, but do not delay. Contact your prescriber immediately or seek urgent obstetric advice. Losartan should be discontinued the same day pregnancy is confirmed [VERIFY].

2. Switch to a pregnancy-safe antihypertensive. Blood pressure control remains important during pregnancy — uncontrolled hypertension itself carries maternal and fetal risks, including pre-eclampsia, placental abruption, and fetal growth restriction. Your clinician will typically start labetalol, nifedipine (extended-release), or methyldopa as a replacement [VERIFY].

3. Obtain detailed ultrasound surveillance. If exposure occurred beyond the first trimester, serial ultrasounds should assess amniotic fluid volume (to detect oligohydramnios), fetal growth, renal anatomy, and skull ossification. A fetal echocardiogram may also be offered [VERIFY].

4. Monitor neonatal renal function at birth. Neonates exposed to ARBs in utero should have serum creatinine, potassium, and urine output closely monitored in the first days of life. Neonatal hypotension may require volume expansion or pressor support [VERIFY].

5. Consider pre-conception planning for future pregnancies. Women with chronic hypertension who are planning pregnancy should transition from losartan (or any ARB/ACE inhibitor) to a pregnancy-compatible agent before attempting conception. This is explicitly recommended by ACOG and by the WHO guidelines on hypertension in pregnancy [VERIFY].

§05Adverse Effects of Losartan in Non-Pregnant Adults and Why They Matter for Pre-Conception Counseling

Understanding losartan's general adverse-effect profile helps contextualize pre-conception counseling. Some adverse effects — particularly those affecting renal function and electrolytes — overlap with the very mechanisms that make the drug dangerous in pregnancy.

Clinical trial data for valsartan, a closely related ARB, confirms that renal impairment, elevated serum creatinine, elevated potassium, and dizziness are the most common class-related adverse effects; patients who experienced these events on ACE inhibitors are likely to experience them with ARBs as well [1]. This underscores that the renal-hemodynamic effects of RAAS blockade — the very effects that benefit adults with hypertension and diabetic nephropathy [7] — are the same effects that prove harmful to the fetus.

For the comprehensive pharmacological profile of losartan, including its physicochemical characteristics, metabolic pathways via CYP3A4 and CYP2C9, and favorable drug-interaction profile, Al-Majed and colleagues (2015) provide a detailed monograph [5]. The absence of clinically significant interactions with hydrochlorothiazide, warfarin, or digoxin [2] is relevant for women on polypharmacy who may need to restructure their medication regimen before conception.

§06Special Populations and Clinical Pearls

Women with diabetic nephropathy

Losartan at 50–100 mg/day has demonstrated significant nephroprotective benefits in patients with type 2 diabetes and proteinuria. In the landmark RENAAL trial, losartan reduced the composite endpoint of doubling of serum creatinine, end-stage renal disease (ESRD), or death by a statistically significant margin compared to placebo (43.5% vs. 47.1%, p = 0.02) [7]. For women of childbearing potential with diabetic kidney disease, the imperative to protect the kidneys must be balanced against the absolute prohibition on ARB use during pregnancy. Pre-conception counseling should address:

• Transitioning to alternative renoprotective strategies (e.g., optimized glycemic control, dietary sodium restriction, and potentially non-dihydropyridine calcium channel blockers, though evidence for nephroprotection is less robust than for ARBs/ACEi).
• The importance of planned pregnancies to minimize the window of fetal ARB exposure.
• Close nephrological monitoring during pregnancy, as diabetic nephropathy itself increases the risk of pre-eclampsia, preterm delivery, and fetal growth restriction [VERIFY].

Losartan's emerging non-cardiovascular uses

Recent research has explored losartan for indications far removed from its original cardiovascular role. Topical losartan at concentrations of 0.2–0.8 mg/mL has shown efficacy in reducing corneal scarring fibrosis by inhibiting TGF-beta signaling through the non-canonical extracellular signal-regulated kinase (ERK) pathway [4][6]. Additionally, losartan has been investigated as an anti-metastatic agent in canine osteosarcoma, where it blocks CCL2-CCR2 monocyte recruitment at doses approximately 10-fold higher than typical antihypertensive dosing [3].

While topical ophthalmic losartan would have negligible systemic absorption compared to oral dosing, no formal studies have assessed its safety in pregnant women. As a precaution, pregnant patients requiring treatment for corneal fibrosis should discuss alternative anti-fibrotic strategies with their ophthalmologist [4][6]. The oncologic use of high-dose losartan [3] is not relevant to typical obstetric populations but illustrates the expanding pharmacological landscape of this molecule.

Breastfeeding considerations

Data on losartan excretion into human breast milk are limited. The drug's relatively low molecular weight (≈461 Da for the potassium salt) and moderate protein binding suggest that some passage into milk is plausible. The AAP (American Academy of Pediatrics) and LactMed database have not classified losartan as clearly compatible with breastfeeding, and many clinicians prefer alternatives with better-established lactation safety profiles (e.g., labetalol, nifedipine, enalapril — noting that ACE inhibitors are generally considered acceptable during lactation despite their pregnancy contraindication) [VERIFY]. Mothers requiring ARB therapy postpartum should discuss the risk-benefit ratio with their clinician.

Pharmacokinetic considerations relevant to washout

Losartan's relatively short half-life (approximately 2 hours for the parent compound, 6–9 hours for the active metabolite E-3174) [2] means that the drug is effectively eliminated within 2–3 days of the last dose. This is reassuring in the context of early first-trimester exposure: if losartan is discontinued promptly upon recognition of pregnancy, drug levels will fall to negligible concentrations rapidly. However, this does not eliminate the risk from exposure that has already occurred, particularly if organogenesis was underway during the exposure window.

No dosage adjustment for losartan is required based on age, sex, race, or mild hepatic impairment [2], but this pharmacokinetic predictability is irrelevant to pregnancy management, where the correct dose is zero.

§07FAQ

Q1: Can I take losartan if I am trying to conceive? A1: No. All major guidelines (ACOG, FDA, EMA, NICE, WHO) recommend that women who are planning pregnancy should switch from losartan — or any ARB or ACE inhibitor — to a pregnancy-compatible antihypertensive before attempting conception. Suitable alternatives include labetalol, nifedipine (extended-release), and methyldopa. Discuss the transition with your prescriber well in advance [VERIFY].

Q2: I took losartan for a few weeks before I knew I was pregnant. Will my baby be harmed? A2: Brief first-trimester exposure, while not ideal, carries a lower absolute risk than prolonged second- or third-trimester exposure. The critical step is to stop losartan immediately upon discovering the pregnancy and switch to a safe alternative. Your obstetrician will likely recommend a detailed anatomy scan at 18–22 weeks and may offer additional fetal surveillance. Most women with early, brief exposure go on to have healthy pregnancies, but individual risk assessment should be done by your healthcare team [VERIFY].

Q3: Why can't I just take a lower dose of losartan during pregnancy? A3: The fetal toxicity of losartan is mechanism-based — it stems from blocking the AT₁ receptor in the fetal RAAS, which is essential for kidney and cardiovascular development. Reducing the dose does not eliminate this pharmacological effect; it merely reduces it to an unpredictable degree. There is no established "safe" dose of losartan in pregnancy, which is why the contraindication is absolute [2][VERIFY].

Q4: Is losartan safe while breastfeeding? A4: There are insufficient data to confirm the safety of losartan during breastfeeding. While the short half-life of losartan and its metabolite [2] suggests limited accumulation, formal studies in lactating women are lacking. Many experts prefer antihypertensives with better-established lactation safety profiles, such as labetalol or nifedipine. If losartan is the only suitable option for the mother, the decision should be made jointly with the prescribing physician and pediatrician [VERIFY].

Q5: Are all ARBs equally dangerous in pregnancy, or is losartan specifically worse? A5: All ARBs share the same mechanism of fetal toxicity — AT₁ receptor blockade disrupting the fetal RAAS. Valsartan [1], candesartan, irbesartan, telmisartan, olmesartan, and azilsartan all carry the same pregnancy contraindication as losartan. There is no evidence that any single ARB is "safer" than another in pregnancy. The same applies to ACE inhibitors, which act on the same system one step upstream. The entire class must be avoided [2][VERIFY].

§08References

[1] Ripley TL. The Annals of Pharmacotherapy 2005. PMID: 15687480. Valsartan in chronic heart failure

[2] Sica DA, Gehr TW, Ghosh S. Clinical Pharmacokinetics 2005. PMID: 16029066. Clinical pharmacokinetics of losartan

[3] Regan DP, Chow L, Das S. Clinical Cancer Research 2022. PMID: 34580111. Losartan blocks osteosarcoma-elicited monocyte recruitment

[4] Dutra BAL, Wilson SE. Journal of Ocular Pharmacology and Therapeutics 2025. PMID: 40238717. Topical losartan in the management of corneal scarring fibrosis

[5] Al-Majed AR, Assiri E, Khalil NY. Profiles of Drug Substances, Excipients, and Related Methodology 2015. PMID: 26051686. Losartan: comprehensive profile

[6] Wilson SE, Dutra BAL, Wahabi K. Journal of Refractive Surgery 2025. PMID: 40488485. Topical losartan in the treatment of stromal scarring after refractive surgery

[7] Carswell CI, Goa KL. Drugs 2003. PMID: 12558462. Losartan in diabetic nephropathy

§09About the author

Dr. Stanislav Ozarchuk, PharmD, has 15 years of clinical pharmacy experience. He writes for PillsCard.com, the international drug encyclopedia.

§10Medical disclaimer

The information provided here is for educational purposes only and is not a substitute for professional medical advice. Always consult a qualified healthcare provider before starting, stopping, or changing any medication.