It is important to understand that the brain contains special receptors that react to a substance in the brain — called endocannabinoid — which is chemically similar to THC. These cannabinoid receptors are widely distributed but highly concentrated in special areas of the brain, such as the hippocampus, amygdala, prefrontal cortex, insula, and anterior cingulate cortex. All of these areas are involved with the control of emotional behavior, learning, and memory.

The cannabinoid receptors are also concentrated in special brain nuclei related to addiction (called the nucleus accumbens and orbitofrontal cortex). When activated, these receptors regulate the release of several neurotransmitters, including dopamine, glutamate, serotonin, and opioids.

By regulating these neurotransmitters, cannabinoids such as marijuana effect the processing of emotional reactions such as responding to rewards, formation of habits, and higher cognitive function (deep thinking and computation).

These cannabinoid receptors also play a critical role in ongoing brain development, which extends from prenatal life until the age of 26 or 27. It is important to appreciate that during brain development, there is a finely tuned increase and decrease of these cannabinoid receptors that is critical for proper wiring of the brain.

The highest concentration of cannabinoid receptors occurs during adolescence — meaning this is a period of great sensitivity to the harmful effects of marijuana. Because the human brain undergoes continued development until age 27, it is until that time vulnerable to toxic substances such as the THC in marijuana.

The effect of exposure to marijuana depends on the timing of the exposure (when one starts using it), the degree of use (dose), and length of exposure (how many years it is used). As noted, exposure before age 18 causes greater harm than later exposure.