Charles Ksir, PhD, Professor of Psychology at the University of Wyoming, and Oakley Ray, PhD, Emeritus Professor of Psychology and Pharmacology at Vanderbilt University, wrote in their 2004 textbook Drugs, Society and Human Behavior:

“There have been…incomplete or poorly controlled reports of potential brain damage from animal research, and it has been possible to dismiss most of them as inconclusive.

However, two experiments on rats, one appearing in 1987 and the other in 1988, gave stronger evidence that THC causes permanent changes in the structure of neurons in the hippocampus.

The doses used were in the range of what a very heavy marijuana smoker might obtain, and the treatment was given to the rats every day for ninety days. It’s not clear what the implications might be for human beings smoking less heavily and only occasionally.

Ironically, some of the nonpsychoactive ingredients in marijuana, including cannabidiol, have been shown to have powerful antioxidant properties that protect brain cells from toxic effects of other chemicals. This effect was strong enough that the NIMH [National Institute of Mental Health] filed a patent in 1988 entitled ‘Cannabinoids as Antioxidants and Neuroprotectants.'”