1. How much THC makes it into breast milk? Can it get a baby “high”? What are the long-term effects on the child? Cannabis is legal in half of the States in the US and legal-ish in 16 other countries around the world. And mothers out there are wondering under which circumstances medical or recreational cannabis use is safe while breastfeeding.

The official guideline is that breastfeeding women should not use cannabis. This is because not enough is known about the topic to state “safe amounts” to mothers. I wasn’t satisfied with that answer, so I kept on digging. Here, I’ll present an overview of the scientific facts, so that you can make up your own mind.
Let’s break the topic down into two questions:
I. How much THC makes it into breast milk?
II. What are the effects on the child?

I recommend reading my post on the Next post on my website. Knowing about how THC—the main active ingredient in cannabis—is metabolized will increase your understanding of the topic. For those in a hurry. We will deal with very small numbers. A quick refresher on the notation and conversion of grams, milligrams and nanograms:
g = gram 1 g = 1,000 mg
mg = milligram 1 mg = 1,000,000 ng
ng = nanogram 1 ng + 0.000000001 g
I. How much THC makes it into breast milk?
“It is generally accepted that all medications transfer into human milk to some degree, although it is almost always quite low. Only rarely does the amount transferred into milk produce clinically relevant doses in the infant.

There is little data published on THC levels in mother’s milk. Most data is from the 1970s and 80s. A study from the 70s found that in monkeys only 0.2% of the ingested THC by the mother gets into the breast milk.
How about humans? Hard to believe, but there is only one study in which two samples of human breast milk were analyzed. The two mothers were heavy users and their milk showed THC levels of 105ng/ml and 340ng/ml. Those two data points are interesting, but don’t answer the question how much THC makes it into breast milk because:
* We don’t know how much cannabis was initially consumed.
* We don’t know how long since the last cannabis consumption.
* With a one time measurement we don’t see how the THC concentration in breast milk behaves over 24 hours.
* Both mothers were heavy users who were consuming marijuana on a daily basis. With regular use, THC accumulates in the body over time, especially in fat. And there is a lot of fat in breast milk. We don’t know which part of the detected THC is due to accumulation and which part is recently consumed THC.
* A sample size of two isn’t representative for a larger population.

True? There is eight times more THC in breast milk than in blood?
This 8:1 ratio has spread across the internet. Beware every author who quotes that number as if it were a fact—they’ve got the story wrong. Here is what actually happened:
Once upon a time in 1982, there was one cannabis smoking mother who agreed to have her blood and breast milk analyzed simultaneously for THC. She was a heavy user, smoking marijuana out of a pipe seven times per day. About one hour after her last consumption the researchers took a sample of her blood and breast milk. It turned out, THC levels in her breast milk were eight times higher than in her blood.
But one sample is not representative for a larger population, nor was it the goal of the study. The study’s goal was to find out if THC was present in human breast milk at all and if so, whether it was going to be metabolized in the child. (The answer to both questions is yes.)
The 8:1 ratio was solely an observation regarding that particular case of the heavy using mother. In my eyes, those researchers are heroes for collecting and sharing that data. But they would surely not approve people quoting that 8:1 ratio out of context.
In short: that study did not find that the THC concentration in milk is 8:1 compared to blood. This ratio was true for one case and one case only.
Here are the claims actually supported by science:
1. THC blood levels have proven to be highly variable across subjects.
2. THC blood levels drop soon after consumption because THC is rapidly taken up by fat tissue. It’s plausible therefore that THC levels would be higher in milk because of its high fat content.
3. Recreational users clear their systems faster.
Quick-and-dirty math
Don’t you want to know how high that baby above got from the THC infused breast milk? I was curious and did the math. Turned out, in one feeding baby experienced about 1.1 percent of what an adult would experience if he ate a weed cookie. In other words:
The baby experienced the equivalent of a space cookie’s crumb.

Very little amounts of THC get transferred into breast milk. Scientific publications give examples of 0.8 percent of the maternal intake of one joint per feeding or a total daily intake of 0.01 to 0.1 milligrams of THC through breastmilk if the mother smokes 1 or 2 cannabis cigarettes per day. The particular circumstances under which those numbers were gathered should be kept in mind though:
Potency of marijuana
Over the past 35 years, cannabis has evolved. The study most authors refer to was conducted in the early 80s, when the average concentration of THC was at 3.2 percent. Until 2008, it had risen to 8.5 percent. In Colorado the average potency of flower (i.e. buds) in 2015 was at 17.1 percent In 2017 18.6 percent.

The few samples analyzed all came from mothers who smoked on a daily basis. With regular use, THC accumulates in fat deposits, from where it is slowly released back into the bloodstream. That’s why the half-life of THC is longer in regular users. Recreational users show lower THC levels in blood and urine and clear it faster from their systems. How fast THC is cleared from specifically breast milk has not been researched yet.

Whether the transferred THC has adverse effects on the child is controversial. (Thin) evidence supports both sides of the argument. Heavy cannabis use in the first month of life might result in inferior motor skills at one year old. But that could also be a result of regular cannabis exposure during the preceding pregnancy.
Weighed against benefits of breastfeeding
On the other hand, the benefits of breastfeeding for mother and child are well documented: optimal nourishment, protection against infections, increased IQ, less obesity, reduced risk of ovarian and breast cancer, the list goes on.
Traditionally, nursing mothers were advised to stop breastfeeding if they wanted to consume cannabis. In recent years, however this position has weakened. The strong health benefits of breastfeeding have led many lactation professionals towards a more moderate benefit/risk calculation. Especially with moderate or occasional use, evidence of detrimental effects of cannabis for the child has yet to be presented. Some doctors even go as far as to say that occasional or low regular use while breastfeeding will probably not be of relevance to the child’s development.

I believe the reasons why a nursing mother would decide to consume cannabis are personal and complex. Furthermore, I believe that most mothers care deeply for their baby and wouldn’t make such a decision lightly.
Personally, I think it’s safe.

This is what I personally found to be useful from studying scientific literature:
(I’m not a doctor and this isn’t medical advice.)
Eating vs. smoking. Ingesting cannabis produces a similar high but at a considerably start small. Have someone sober around who can take over with the child if needed. With varying quality of product, effects can be unpredictable. Especially casual users can experience paranoia or impaired judgement. Caring for a child in such a case can be difficult.
timing matters. During pregnancy and the first three months of life, a baby’s developing organism is most vulnerable.

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