Can I Smoke Marijuana While Breastfeeding?

Is there any data or studies about breastfeeding and marijuana use and the potential harms to an infant?
—Aubrey

I get this question a lot — as marijuana legalization has spread widely through the U.S., more people are using marijuana before pregnancy, and an obvious question is then whether you can use it during pregnancy or breastfeeding. I wrote about the pregnancy question in the updated edition of Expecting Better, and I’ve touched before on breastfeeding in this post. There, I focused on studies that look at developmental outcomes for kids who are exposed to THC during breastfeeding versus not, and basically argue that we do not have good enough data to say anything conclusive.

When it comes to breast milk (in contrast to pregnancy), there is another way into this question, which is to test the milk directly to see how concentrated the cannabis products are and for how long. (This is the same approach we can take with alcohol, where we see low concentrations overall in milk and they clear completely within a few hours.)

Recently, a helpful reader (who asked not to be named, but I will say that this person is a postdoc scientist at an elite university who works in a related area) sent me a long email pointing to some relatively new evidence from testing breast milk directly after cannabis consumption. What do we see?

The main paper is here. The authors enrolled 20 subjects, nursing mothers who reported using cannabis recreationally, and tested their milk. There are a few findings of note. First: there is a positive (though not perfect) correlation between THC concentration in the blood and in milk; on average, the concentration in the milk is about seven times as high.

Second: THC concentrations in breast milk fell precipitously over time. They were high, on average, for the first 5 to 10 hours after use, and remained elevated through about 15 hours. Samples that were tested more than 15 hours after use showed little or no THC concentration in milk. In a second paper, with eight women studied using a similar approach, the fadeout was faster — low concentrations by about five hours. This second paper has the advantage of doing a true challenge — they gave the cannabis, and tested blood over time. The first relied on self-reports about timing.

One can do some calculations based on this, which suggest that the dose of cannabis an infant would receive through milk is fairly high (perhaps a third of an adult dose, weight-adjusted) if nursing occurred immediately after smoking, but lower (less than 1% of an adult dose) if nursing occurred hours later. (This calculation uses other data on concentration of THC in blood immediately after smoking, adjusted for the 8 times higher concentration in milk than blood, assumes a 3.5 ounce breastmilk feed and adjusts for an adult being 15 times as large as a baby. You could alter these assumptions!)

The piece that is missing here is an understanding of the possible risks of cannabis consumption to babies; that is not part of this calculation. This leaves us, I think, with remaining questions and — in my mind — remaining reason for caution about use at all. But there is an important, concrete takeaway: If you do choose to consume cannabis while breastfeeding, waiting 5 to 10 hours after consumption to nurse is a very good idea.