We continue to see more and more women of reproductive age using marijuana in various forms. While marijuana is viewed by some as a safe alternative for the treatment of various maladies associated with pregnancy, including morning sickness and postpartum depression, there is no data to support the use of marijuana for the management of these conditions.  

We have data to indicate that the use of cannabis during pregnancy may negatively affect fetal growth and brain development.  There is considerably less information on the use of cannabis-containing products in women who are breastfeeding and on how the use of cannabis may affect nursing infants.  What we do know, however, is that no matter how marijuana/cannabis is consumed (smoking, vaping, or ingesting), its byproducts can be found in the breast milk.  Figuring out how much is passed into the breast milk is complicated because how women use cannabis varies considerably. The pharmacokinetics and clearance may differ depending on whether the cannabis is inhaled or ingested; however,  both cannabidiol (CBD) and the psychoactive component, delta-9-tetrahydrocannabinol or THC, have been detected in breast milk.  

In a recent study, Wymore and colleagues screened 394 women who had recently given birth and enrolled 25 women who screened positive for marijuana use during pregnancy but were planning to abstain from marijuana while breastfeeding.   The researchers collected data on self-reported marijuana usage and measured levels of THC in maternal plasma and breast milk samples several times a week. 

In all 25 participants, THC was detectable in breast milk throughout the six week duration of the study.   The median THC concentration in breast milk was 3.2 ng/mL  (IQR, 1.2-6.8) within the first week after delivery, increased to 5.5 ng/mL (IQR, 4.4 – 16.0) at 2 weeks, and declined to 1.9 ng/mL (IQR, 1.1 – 4.3) at 6 weeks. (The IQR or interquartile range (IQR) give us a sense of where the middle 50% of THC levels fall and indicates how widely spread out the values are. ) 

While this finding suggests that THC levels continue to be detectable six weeks after delivery, it must be noted that only 12 of the 25 women self-reporting abstinence from marijuana were, in fact, abstinent based on maternal plasma levels of THC.  In a subset of 7 women who abstained more than 5 weeks, the researchers estimated the mean half-life of THC in breast milk to be 17 days (SD 3.3).  Based on this estimate, they calculated that it would be possible to detect THC in breast milk for longer than 6 weeks.

In addition, the researchers were able to calculate a milk:plasma partition coefficient for THC which was approximately 6:1 (IQR, 3.8:1 – 8.1:1).  Milk:plasma ratios give us a sense of how easily a compound passes from the mother’s bloodstream into the breast milk and can be used to estimate the amount of exposure through breast milk.  Most M:P ratios for drugs commonly used in breastfeeding women are around 1 or less than 1; thus, an M:P ratio for THC of 6 is high and suggests that levels of THC in the breast milk may be higher than in the mother’s bloodstream.  

Is Breast Milk a Reservoir for THC?

Cannabis concentrations in the breast milk are variable and are related to maternal dose and the frequency of use.  However, there are some things that make cannabis a little different than alcohol or other recreational drugs. Cannabis and its byproducts are very fat-soluble or lipophilic.  Because in women the percentage of body fat is 25-30%, there is a large reservoir for the storage of THC.  What this means is that it takes much longer for cannabis to leave one’s system, compared to substances like alcohol.  Furthermore, there is an especially long washout period in those who are daily users. Long after the psychoactive effects have faded, THC and its metabolites can be detected in blood, urine, and breast milk.  

These findings of the Wymore are consistent with previous studies measuring THC in breast milk which observed a duration of detection ranging from 6 days to greater than 6 weeks after using cannabis.  The longevity of THC in the breast milk may be related, in part, to the extremely high fat content of breast milk and the lipophilic nature of THC, so that the breast milk “traps” the THC, in a sense acting like a reservoir for THC storage.

While some women using cannabis may believe that they can limit exposure by adjusting when they nurse their infant or by “pumping and dumping” after using cannabis, the findings of the Wymore study indicate that this approach would not be an effective means of avoiding or minimizing exposure.  With an estimated half-life of 17 days, it would take about a week to see a 50% reduction in the amount of THC in the breast milk.     

Thus the only means of minimizing exposure to THC in the nursing infant is to abstain.  


Ruta Nonacs, MD PhD


Wymore EM, Palmer C, Wang GS, Metz TD, Bourne DWA, Sempio C, Bunik M.  Persistence of Delta-9-Tetrahydrocannabinol in Human Breast Milk.JAMA Pediatr. 2021 Mar 8. 

THC Persists in Breast Milk 6 Weeks After Quitting Cannabis (Medscape)

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