Author: Jacqueline Gerhart, PhD on June 30, 2026 
Figure. Median nirmatrelvir (A) and ritonavir (B) plasma and breast milk concentration-time profiles on Day 2 after administration of multiple oral doses of nirmatrelvir/ritonavir 300 mg/100 mg twice daily.
Pregnant or lactating women with COVID-19 are at increased risk of developing severe disease or complications, such as hypertension, preeclampsia, or a need for intensive care admission.1,2 However, because of perceived safety concerns regarding infant exposure, breastfeeding women were excluded from the pivotal phase 2/3 nirmatrelvir/ritonavir trials, resulting in a lack of clinical data in this population.
To address this clinical knowledge gap, we recently published results of a prospective phase 1 study in healthy women who were at least 12 weeks postpartum and were actively breastfeeding or expressing breastmilk.3 Eight women received three oral doses of nirmatrelvir/ritonavir 300 mg/100 mg over 2 days (i.e., steady-state dosing), and drug concentrations were measured in maternal plasma and breast milk.
The concentrations of nirmatrelvir and ritonavir that transferred to breast milk were consistently lower than that transferred to maternal plasma (Figure). The geometric mean absolute daily dose that an infant would receive (assuming a standard infant breast milk consumption of 150 mL/kg/day) was 0.1595 and 0.0057 mg/kg/day for nirmatrelvir and ritonavir, respectively. This represents 1.8% and 0.19% of the bodyweight-normalized maternal dose (sometimes known as the relative infant dose) of nirmatrelvir and ritonavir, respectively, both of which are well below the US National Institutes of Health (NIH) acceptability threshold of 10% for infant exposure.4 These findings combined with the acceptable safety profile for nirmatrelvir/ritonavir demonstrated a favorable benefit-risk profile for the use of nirmatrelvir/ritonavir in lactating women.
References
1. Villar J, Ariff S, Gunier RB, et al. Maternal and neonatal morbidity and mortality among pregnant women with and without COVID-19 infection: the INTERCOVID multinational cohort study. JAMA Pediatr. 2021;175(8):817-826.
2. Xu K, Sun W, Yang S, et al. The impact of COVID-19 infections on pregnancy outcomes in women. BMC Pregnancy Childbirth. 2024;24(1):562.
3. Gerhart J, Cheruvu N, Pelletier K, et al. A phase 1 multiple-dose, open-label, pharmacokinetic study of nirmatrelvir/ritonavir in healthy lactating women. Clin Transl Sci. 2026;19(5):e70552.
4. National Institute of Child Health and Human Development. Drugs and Lactation Database (LactMed®). https://www.ncbi.nlm.nih.gov/books/NBK547442/. Accessed April 1, 2025.
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