Is Montelukast Safe in Pregnancy and Breastfeeding?

Montelukast is generally considered compatible with pregnancy and breastfeeding based on the available human evidence, though no medication used during pregnancy carries a guarantee of zero risk. Multiple large studies, including a nationwide cohort covering hundreds of thousands of pregnancies, have found no increased rate of major birth defects among women who took montelukast while pregnant. The drug also passes into breast milk in only trace amounts. That said, the picture is more nuanced than a simple “safe” or “not safe,” and understanding why researchers are cautiously reassuring rather than emphatically certain matters if you are weighing whether to keep taking it.

Why Uncontrolled Asthma in Pregnancy Is the Bigger Concern

Before evaluating montelukast specifically, it helps to understand what happens when asthma goes poorly controlled during pregnancy. The risks are not abstract. Poorly managed asthma raises the chances of preterm birth, low birth weight, perinatal death, pre-eclampsia, and gestational diabetes.¹ Asthma exacerbations during pregnancy, where symptoms flare badly enough to need urgent treatment, are independently linked to pre-eclampsia.² Pre-eclampsia alone can be life-threatening for both mother and baby.

This is the context that every conversation about asthma medication in pregnancy has to sit inside. The question is not “does the drug have any theoretical risk” but “does the risk of the drug outweigh the known, well-documented harm of letting asthma run unchecked?” For montelukast, the answer from current evidence is that continuing the drug when it is already working appears safer than stopping and losing asthma control.³

What Large Studies Say About Birth Defects

The central worry with any drug taken early in pregnancy is whether it could cause structural problems in the developing baby. Several population-level studies have looked at this directly for montelukast, and none have found a meaningful signal.

A nationwide population-based cohort study compared women who used various asthma controller medications during the first trimester, including montelukast alone, inhaled corticosteroids alone, and several combination regimens, against women with asthma who did not use controller medications. None of the five medication groups showed an increased risk of major congenital malformations.⁴ A Danish population study found similar results: adjusted odds ratios for congenital anomalies were not significantly elevated among montelukast-exposed pregnancies compared to the general population.⁵

A retrospective insurance claims analysis covering more than 277,000 pregnancies linked to live births, with about 1,535 montelukast-exposed infants, found that rates of congenital anomalies were similar in the montelukast group, the inhaled corticosteroid group, and the general population group. The researchers concluded there was no epidemiologic evidence supporting a causal relationship between montelukast and birth defects.⁶

The Limb-Reduction Defect Question

If you have ever searched for montelukast and pregnancy, you may have come across mentions of limb-reduction defects, a type of birth defect where part of a limb does not form completely. This concern traces back to a specific cluster of reports that emerged from postmarketing surveillance.

Between montelukast’s initial approval in 1997 and June 2006, Merck’s global surveillance system identified six reports of limb-reduction defects in infants born to women who had taken the drug during pregnancy. Three independent experts in congenital malformations reviewed the cases and noted they shared a broadly similar presentation. However, these experts also concluded there were no compelling data pointing to a biological mechanism by which montelukast could cause such defects. Animal studies at doses roughly 100 times the maximum recommended human dose showed no signs of these or other birth defects. And no additional limb-reduction reports were received after June 2006.⁶

The large retrospective cohort study of over 1,500 montelukast-exposed infants that followed found zero cases of limb-reduction defects similar to those six postmarketing reports. This does not mean the original reports were wrong, but it strongly suggests the cluster was a coincidence rather than a drug effect. Limb-reduction defects occur in the general population at a low background rate, and a handful of cases among many thousands of exposures is statistically expected with or without any particular drug. The researchers and expert reviewers ultimately agreed the data did not support a causal link.

Miscarriage Risk

Spontaneous abortion, or miscarriage, is another outcome that understandably concerns pregnant people taking any medication. A multicentre prospective study tracking 180 montelukast-exposed pregnancies reported 20 spontaneous abortions, along with 160 live births and 2 elective terminations.⁷ At first glance, that number might seem high, but spontaneous abortion rates in the general population are estimated at about 10 to 20 percent of recognized pregnancies, and the rate in this study fell within that range.

A more recent systematic review and meta-analysis that pooled data across multiple studies found no clear association between montelukast use and spontaneous abortion, with an odds ratio very close to 1.0. The authors noted the result was inconclusive because of high variability between studies and called for more research.⁸ In practical terms, the existing evidence does not point to montelukast raising miscarriage risk, but the data are not plentiful enough for researchers to say so with complete confidence.

What Happens During Breastfeeding

The other half of the question, whether montelukast is safe while nursing, has been studied directly. A pharmacokinetic study measured montelukast concentrations in the breast milk of lactating women who were taking the drug. Average levels in milk were about 5.3 nanograms per milliliter, and the peak concentration was about 9.7 nanograms per milliliter at roughly four hours after the dose. The relative infant dose, a standard measure of how much drug a breastfed infant would receive relative to the mother’s weight-adjusted dose, was 0.68%.⁹

In lactation pharmacology, a relative infant dose under 10% is widely considered the threshold below which a drug is generally regarded as compatible with breastfeeding. At under 1%, montelukast falls well beneath that cutoff. The infant would be exposed to a tiny fraction of the dose, making clinically meaningful effects on the baby extremely unlikely. This is one of the more reassuring data points in the entire montelukast-and-reproduction evidence base.

What Guidelines Actually Recommend

Clinical guidelines have converged on a practical recommendation: if you are already taking montelukast and it is controlling your asthma effectively, you can continue it during pregnancy. Both the Asthma and Pregnancy Working Group and the British Thoracic Society guidelines suggest that leukotriene receptor antagonists can be continued when treatment response has been demonstrated before pregnancy, with montelukast having the most evidence behind it.¹⁰

The emphasis on “already established” is deliberate. Starting montelukast for the first time during pregnancy is less commonly recommended, not because the drug is thought to be more dangerous when newly started, but because inhaled corticosteroids remain the preferred first-line controller for asthma in pregnancy. If your asthma is not yet well controlled and you are newly pregnant, most clinicians will reach for an inhaled corticosteroid first and add montelukast only if needed. If you are already on montelukast and doing well, switching to something else just because you became pregnant introduces a new risk: losing control of your asthma during the transition.

The Neuropsychiatric Warning and Pregnancy

In 2020, the FDA added a boxed warning to montelukast regarding neuropsychiatric side effects, including mood changes, agitation, sleep disturbances, and in rare cases suicidal thoughts. This warning applies to all patients, not specifically to pregnant ones, but pregnancy is already a period of heightened emotional vulnerability for many people, which makes the warning worth thinking about.

The guideline reviews acknowledge this concern and note that montelukast should be monitored closely in patients who are considered vulnerable to neuropsychiatric complications.¹⁰ If you have a history of depression, anxiety, or other mental health conditions, it is worth having a frank conversation with your provider about whether montelukast is still the best option for you during pregnancy or whether an alternative controller would work just as well without that particular risk profile. The boxed warning does not mean the drug is unsafe overall; it means certain people need closer follow-up while taking it.

How Montelukast Compares to Other Asthma Controllers

Montelukast works differently from inhaled corticosteroids. It is a leukotriene receptor antagonist, meaning it blocks specific inflammatory molecules involved in both airway narrowing and inflammation. It is taken as a daily pill rather than an inhaler.¹¹ For some people, particularly those who also have allergic rhinitis, this dual mechanism makes it a convenient single treatment. For others, it works best as an add-on to an inhaled corticosteroid when the inhaler alone is not enough.

In pregnancy, inhaled corticosteroids, particularly budesonide, have the longest track record of safety data and are considered the first-line controller. This does not mean they are safer than montelukast in any proven way; it means more pregnancies have been studied with inhaled corticosteroids, so the confidence level is higher. The nationwide cohort study that compared multiple controller strategies found that none of them, including montelukast, raised the risk of major congenital malformations above the background rate.⁴ So the choice between them during pregnancy is more about what is already working for you than about one being categorically safer than the other.

Practical Decisions for People Who Are Pregnant or Planning Pregnancy

If you are currently taking montelukast and discover you are pregnant, do not stop the medication abruptly without talking to your provider. Suddenly dropping asthma control can trigger exacerbations, and as discussed earlier, exacerbations carry their own serious risks. The evidence consistently suggests that continuing montelukast when it is already effective is a reasonable choice.

If you are planning a pregnancy and currently take montelukast, this is a good time to have a preconception conversation with your healthcare provider. Questions worth discussing include:

Asthma severity: Is your asthma well controlled on montelukast alone, or would you need it in combination with other medications?
Mental health history: Have you experienced mood changes, sleep disruption, or other neuropsychiatric symptoms on the drug?
Alternative options: If you are on montelukast mainly for allergic rhinitis rather than asthma, are there pregnancy-compatible nasal treatments that could replace it?
Step-down possibility: Could you trial a switch to an inhaled corticosteroid alone before conception to see if that provides adequate control?

These are not urgent questions. There is no reason to panic if you took montelukast in early pregnancy before knowing you were pregnant. The large cohort studies specifically included first-trimester exposures and still found no increased risk of malformations.

Gaps in the Evidence

It is worth being honest about what the research has not yet nailed down. Most of the available studies are observational, meaning they follow women who happened to take montelukast and compare their outcomes to women who did not. This design can detect large safety signals reliably, but it struggles with rare outcomes and subtle effects. Randomized controlled trials, the gold standard for causation, are essentially impossible to conduct for drug safety in pregnancy for ethical reasons. No one is going to randomly assign pregnant women to take a medication versus a placebo just to see what happens.

The miscarriage data illustrate this limitation well. The meta-analysis found no clear link between montelukast and spontaneous abortion, but the variability across individual studies was high enough that the authors could not rule it out definitively.⁸ This does not mean there is a hidden risk lurking; it means the sample sizes are not yet large enough to say with the kind of statistical certainty researchers prefer.

Similarly, while the congenital malformation data are consistently reassuring across multiple large studies, rarer defect types would require even larger sample sizes to detect. The limb-reduction concern emerged from just six cases worldwide, a number so small that it was statistically impossible to distinguish from background noise, which is exactly what larger studies subsequently confirmed. For very rare outcomes, the absence of a signal in studies of thousands of pregnancies is meaningful, but it is not the same as mathematical proof of zero risk. No drug and no exposure in pregnancy, including things as mundane as common foods, comes with that proof.

Montelukast for Allergic Rhinitis During Pregnancy

Montelukast is approved not only for asthma but also for allergic rhinitis, and some women take it primarily for seasonal or year-round nasal allergies rather than for breathing difficulties.¹¹ This matters because the risk-benefit calculation shifts depending on why you are taking the drug. For asthma, where poorly controlled disease carries genuine risks to the pregnancy, the case for continuing montelukast is strong. For allergic rhinitis alone, the risks of stopping are mainly discomfort rather than danger, so the conversation with your provider might go differently.

Nasal congestion is already common during pregnancy due to hormonal changes in blood flow and mucous membranes. Some women find their allergy symptoms worsen, while others experience improvement. If you are taking montelukast solely for nasal allergies, your provider might suggest trying intranasal corticosteroid sprays or saline irrigation first, since these deliver medication locally with minimal systemic absorption. But if montelukast is the only thing that adequately controls your symptoms and those symptoms are affecting your sleep, eating, or quality of life, the safety profile described above still applies. The drug is not treated differently depending on whether the prescription says “asthma” or “allergic rhinitis.”

1
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