Researchers gave three joints (marijuana) per day, five days a week to monkeys, which is less exposure than many humans give themselves. After six months the monkeys exhibited chronic symptoms ranging from listlessness to irritability. No big deal. But then the monkeys were no longer given the marijuana, and they did not return to normal. Even after eight months, their brain wave patterns were quite abnormal. When the monkey’s brains were examined with a microscope, physical damage was evident.
The THC [tetrahydrocannabinol] content of marijuana common during the 1960’s was typically one percent. THC is the active ingredient. Marijuana growers worked constantly to improve the quality of their weed and now it contains up to 14 percent THC. The monkeys received 3 percent.