Yes, weed affects breast milk. THC, the main psychoactive compound in cannabis, concentrates in breast milk at levels several times higher than in the mother’s blood, and it can linger there for weeks after the last use. Both major medical organizations in the U.S. that guide pregnancy and pediatric care advise against cannabis use while breastfeeding.
Why THC Concentrates in Breast Milk
THC is highly fat-soluble, and breast milk is rich in fat. That combination creates a one-way trap. After you use cannabis, THC moves quickly from your blood into fatty tissues throughout your body, including breast tissue. Once it crosses into milk, it becomes partially ionized (changes its chemical charge), which prevents it from passively diffusing back into your bloodstream. It essentially gets stuck there.
The result is that THC levels in breast milk are consistently higher than in the mother’s blood. Studies measuring paired samples of plasma and breast milk have found milk-to-plasma concentration ratios as high as 8 to 1. A pharmacokinetic modeling study published in Pharmaceutics calculated a milk-to-plasma ratio of about 3.3 on average, with peak THC concentrations in milk roughly double those in plasma (155 ng/mL in milk versus about 70 ng/mL in plasma).
THC also builds up in your body’s fat stores over time. Adipose tissue can hold THC at concentrations up to 10,000 times higher than what’s circulating in your blood. Because those fat stores slowly release THC back into circulation, the compound can appear in breast milk for weeks after you stop using cannabis. This is fundamentally different from alcohol, which clears from milk as it clears from blood within a few hours.
How Much THC Reaches the Baby
Modeling research estimates that a breastfed infant receives between 0.34% and 0.88% of the mother’s THC dose through milk. That may sound small, but infants process drugs very differently than adults. Their livers are immature, their body fat percentage is different, and their brains are developing rapidly. A one-month-old infant exclusively breastfed by a mother who smokes cannabis once daily was predicted to have a plasma THC level roughly 0.2% of the mother’s, which, while low, represents continuous low-grade exposure during a critical window of brain development.
The relative infant dose also varies by the baby’s age. The modeling study found it peaked around three months (0.71%) and dropped to about 0.39% by twelve months, likely reflecting changes in how much milk the baby consumes relative to body weight as solid foods are introduced.
Effects on the Baby
Research on what this exposure actually does to breastfed infants is limited, and that uncertainty is itself part of the concern. There is no comprehensive body of research confirming that low-level THC exposure through milk is safe for a developing brain. The FDA has noted that high doses of CBD in pregnant animals caused problems with the reproductive system of developing male fetuses, raising questions about cannabinoid exposure more broadly during early development.
What is known is that THC activates the same receptor system (the endocannabinoid system) that plays a key role in brain development during infancy. Exposing a developing nervous system to an outside source of cannabinoids during this period is a theoretical risk that researchers have not been able to rule out. The lack of clear evidence of harm is not the same as evidence of safety, particularly for a substance that accumulates in the exact tissue (fat-rich milk) that an infant depends on exclusively for months.
Effects on Milk Supply
Cannabis may also affect how much milk you produce. THC can suppress prolactin, the hormone that drives milk production. A single use has been shown to lower prolactin levels acutely in non-lactating women, though the picture gets murkier with long-term use. Some chronic users show no change in prolactin, while others actually develop abnormally high levels.
In practice, cannabis-using mothers have reported lower milk production during the first six weeks postpartum compared to non-users. That said, the majority of cannabis-using mothers in one survey (about 86%) said they noticed no difference in their supply when using versus not using. The disconnect may reflect differences in how often or how much cannabis each person uses, or it may simply be hard to self-assess milk volume accurately.
Cannabis use is also linked to shorter breastfeeding duration overall. A Colorado survey found that only 58% of postpartum cannabis users breastfed for nine or more weeks, compared to 79% of non-users. A separate study of nearly 5,000 postpartum women found that marijuana users were more likely to breastfeed for less than eight weeks. It’s worth noting that cannabis-using mothers in these studies were also more likely to smoke cigarettes and experience postpartum depression, both of which independently affect breastfeeding duration.
Why “Pump and Dump” Doesn’t Work Here
With alcohol, pumping and discarding milk after drinking can effectively reduce what your baby is exposed to, because alcohol clears from milk as your blood alcohol drops. THC does not work this way. Because it is stored in body fat and slowly re-released over days to weeks, pumping and dumping a few sessions’ worth of milk does not meaningfully reduce your baby’s exposure. Colorado’s public health guidance specifically notes that this strategy is not considered effective for cannabis because the compounds persist in the body long after use.
CBD Products Carry Similar Unknowns
CBD is also fat-soluble with a small molecular weight, which means it likely transfers into breast milk through the same mechanism as THC. There is no comprehensive research on what CBD does to a breastfed infant. The FDA has flagged additional concerns: CBD products are frequently contaminated with THC or other substances, CBD is associated with liver effects and extreme drowsiness in clinical studies, and it can interact with other medications. If you’re using CBD for anxiety, pain, or sleep during the postpartum period, the same caution applies as for THC-containing cannabis.
Secondhand Smoke Is a Separate Risk
Even if you’re not breastfeeding, smoking or vaping cannabis near your baby exposes them to THC through secondhand smoke. The CDC advises that parents and caregivers avoid smoking marijuana products around babies or children. This is a distinct exposure pathway from breast milk and adds to the total amount of THC an infant could absorb. Edibles eliminate this particular risk but do not change what enters your milk.