As marijuana is legalized in more states, questions about its safety and the health consequences of cannabis use are becoming mainstream. A new study published in PNAS finds that use of cannabis by pregnant women can have implications for the neural development of her child and that some of the consequences continue into adulthood, So, like alcohol, another recreational drug that is legal in the US, marijuana is likely best avoided by pregnant women.

The most prominent active ingredient in marijuana is a compound known as THC, which interacts with the naturally occurring cannabinoid receptors in the nervous system. Cannabinoid receptors are known to play an important role in the regulation of brain development, and this paper examines the influence of a prenatal THC exposure on the maturation of pathways regulated by these receptors.

The study examined prenatal cannabis consumption in mice, with the aim of identifying the mechanisms responsible for cannabis-related changes in brain function. During the study, pregnant mice were exposed to daily injections of THC or injections of a control liquid. Then the offspring were run through a battery of behavioral tests. The animals’ brains were also examined closely using immunoflouresence and confocal microscopy. Embryonic brain tissue from some litters was also collected and checked for irregularities.

The data from the study shows that exposure to THC disrupted the brain’s native cannabinoid receptors, and this disruption resulted in abnormalities of the cortex that persisted into adulthood. These changes had long-term functional consequences for the offspring. Mice with prenatal exposures suffered deficits in neuron connectivity and experienced alterations in motor function related to other abnormalities.

Mice with embryonic exposure to THC also exhibited increased susceptibility to seizures in adulthood, even after natural cannabinoid receptors in their neurons returned to normal levels following their THC exposure. Therefore, the consequences of prenatal exposures to THC were lifelong for these mice.

These findings have important implications beyond just recreational marijuana use. Cannabinoid-based drugs similar to THC are used to treat a variety of medical problems, including seizures. Given these new findings, it will be important to investigate the long-term consequences of prenatal exposure to these drugs. Careful dosing and regulation may be needed for pregnant women.

This study isn't conclusive about effects in humans; it was done in mice, and it used a regular dose that may not reflect human use habits. But at a bare minimum, these findings suggest we should be avoiding recreational cannabis use during pregnancy. Perhaps someday soon legal marijuana will come with a “do not consume while pregnant” warning, just like alcohol does.

PNAS, 2015. DOI: 10.1073/pnas.1514962112 (About DOIs).