There is no evidence that marijuana, unlike alcohol, causes any permanent brain damage.

Prohibitionists frequently claim that marijuana kills brain cells, or that smoking pot “makes you stupid”.  Where do these claims come from? Are they true? There was a researcher in the 1970’s by the name of Robert G. Heath who claimed he found some brain damage in three monkeys he had dosed heavily, (the equivalent to smoking 30 joints a day), with marijuana. 

His experiment was never replicated, and since then several other better controlled and much larger studies found no physical alteration at all in monkeys given marijuana daily for a year. There have also been several studies of humans that confirm this. There is no evidence that marijuana, unlike alcohol, causes any permanent brain damage. (#1)

There is however, some evidence that frequent marijuana users suffer persistent deficits in short-term memory even after a few months of abstinence. In other words it is probably not a good idea to use large amounts of marijuana while you are trying to memorize or learn new things but there is no permanent damage and short-term memory will return.

There is a study done at Harvard that is frequently cited as evidence that marijuana does cause brain damage in young people. The researchers conducted MRI brain scans of 20 marijuana smokers and 20 who did not smoke marijuana. The study found differences in the brains of the users and nonusers, but it did not establish that marijuana use caused the variations or that they had any functional significance. (#2)

Additional research has been done by the University of Alabama comparing the brains of twins – where one regularly consumed marijuana and the other did not. The study lasted nearly a decade, and showed no difference in their IQ scores as a result of marijuana consumption. In a recent investigative story on the health effects of marijuana use, National Public Radio reported that “the research suggests if you don't start young and don't use marijuana often, there's not much evidence of permanent harm to the brain.” (#3)

The study lasted nearly a decade, and showed no difference in their IQ scores as a result of marijuana consumption.

 Legalizing and regulating marijuana would help those who choose to use it do so more responsibly and safely. Labels would explain the strength of each product, government regulations would control the production to prevent the addition of any harmful chemicals, and age restrictions on purchasing would help keep it out of the hands of young people whose brains are still developing. If marijuana is to be made safer, the only choice is legalization.

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