Check out this great article from Sarah Biren and The Hearty Soul!
The concept of smoking, vaping, or using cannabis edibles is becoming increasingly mainstream. This drug is legal for medical use in 33 states and for recreational use as well in 11 states. However, the current system for classifying illegal drugs is flawed and outdated. Marijuana has been categorized in the restrictive class of “high potential for abuse,” “no currently accepted medical use,” and a “lack of accepted safety.” This restricts patient access to the medicinal drug and proper research. 
That is one reason why there’s limited information on marijuana and how it affects the brain.