evidence: Marijuana in breast milk was first described in 1982, about the same time as the first long-term studies of marijuana use in pregnancy were beginning. Those studies were limited in scope, and marijuana has changed a lot since then. THC was about 4% per joint. Today, it’s more than 20%. [...] "It’s not surprising that we detected THC in the breast milk," Dr. Bunik says, "given that it’s a lipophilic substance and lives in fatty tissue. But it lasts much longer than we thought it would. I think we had hoped to prove a shorter duration of exposure to the psychoactive component." Importantly, THC was more concentrated in breast milk than in plasma or urine, which suggests those testing sources aren’t as reliable a foundation for breastfeeding recommendations. Even more surprising was the range of THC concentration in breast milk. Some participants had very small amounts, and some had 10 to 100 times higher, which is likely related to patterns of marijuana use, a person’s BMI or their metabolism.