Montelukast does not appear to significantly increase the risk of birth defects when used during pregnancy. A 2024 meta-analysis pooling data from multiple studies found no statistically significant increase in major congenital malformations, with a risk ratio of 1.13 (meaning essentially no meaningful difference from baseline). That said, the evidence is more nuanced than a simple “safe” or “not safe,” and the decision depends on how well your asthma is controlled without it.
Birth Defect Risk
Every pregnancy carries a baseline 3 to 5 percent chance of a birth defect regardless of medication use. The key question is whether montelukast pushes that number higher, and the available data suggests it does not. Medical record reviews covering thousands of montelukast-exposed pregnancies have not found an increased rate of birth defects. Additional studies examining a combined total of over 200 exposed pregnancies reached the same conclusion.
The most rigorous look at this question, a 2024 systematic review and meta-analysis, calculated a pooled risk ratio of 1.13 with a confidence interval that crossed 1.0, meaning the slight numerical increase was not statistically significant and could easily be due to chance. There was also no inconsistency across the included studies, which strengthens the finding.
Effects on Birth Weight and Gestational Age
Where the picture gets a bit more complicated is with birth weight and timing of delivery. One well-known study from Canada found that infants exposed to montelukast had a lower average birth weight (about 3,214 grams, or roughly 7 pounds 1 ounce) and a slightly shorter gestational age at birth (37.8 weeks) compared to healthy pregnant women not taking any potentially harmful medications. However, when those same montelukast-exposed pregnancies were compared to women with asthma who were treated with other medications, the differences largely disappeared.
This is an important distinction. Uncontrolled asthma itself is associated with lower birth weight, preterm delivery, and preeclampsia. So it can be difficult to separate the effect of the drug from the effect of the underlying disease. A subanalysis of women who continued montelukast through the end of pregnancy found the only remaining statistical difference was in birth weight across the three groups, and again, the disease itself likely plays a role.
Some studies have also flagged a possible association with preeclampsia and preterm delivery (before 37 weeks), but at least one study found no difference in birth weight between montelukast-exposed babies and babies exposed to other asthma treatments. Miscarriage rates have not been shown to increase with montelukast use.
Where Montelukast Fits in Asthma Treatment
Montelukast is not typically the first medication tried for asthma during pregnancy. Inhaled corticosteroids remain the preferred controller therapy because they have the longest track record of safety in pregnant women. Montelukast is considered a second-line or add-on option, used when inhaled steroids alone are not enough to control symptoms.
Among medications in its class (leukotriene receptor antagonists), montelukast is the preferred choice during pregnancy because it has been studied more extensively than alternatives. If you were already taking montelukast before becoming pregnant and your asthma is well controlled, your provider will weigh the risks of switching medications or stopping treatment against the risks of uncontrolled asthma, which include oxygen deprivation for the baby, preeclampsia, and preterm birth. Poorly controlled asthma generally poses a greater threat to pregnancy outcomes than montelukast does.
The FDA’s Neuropsychiatric Warning
In 2020, the FDA added its most serious warning (a boxed warning) to montelukast for neuropsychiatric side effects, including agitation, depression, sleep disturbances, and in rare cases suicidal thoughts. These effects have been reported in patients with and without any prior history of mental illness, and some cases occurred even after stopping the medication.
This warning applies to all patients, not specifically to pregnant women. But pregnancy itself can increase vulnerability to mood changes and anxiety, so it is worth being aware of. If you have a history of depression or anxiety, this is especially relevant to discuss with your provider before starting or continuing montelukast.
Breastfeeding After Delivery
Montelukast passes into breast milk in very small amounts. In a study of seven women taking the standard 10 mg dose, the average milk concentration was 5.3 micrograms per liter, resulting in an estimated infant dose of just 0.68 percent of the mother’s weight-adjusted dose. For context, anything under 10 percent is generally considered low exposure. This suggests minimal risk to a breastfeeding infant, though formal recommendations from some sources still advise caution due to limited data overall.