Last updated on: 5/30/2008 10:20:00 AM PST
Does Marijuana Use Permanently Damage the Human Brain?



PRO (yes)

The National Institute on Drug Abuse (NIDA) noted in its online article "NIDA InfoFacts: Marijuana" (revised Mar. 2004):

"When someone smokes marijuana, THC rapidly passes from the lungs into the bloodstream, which carries the chemical to organs throughout the body, including the brain.

In the brain, THC connects to specific sites called cannabinoid receptors on nerve cells and influences the activity of those cells. Some brain areas have many cannabinoid receptors; others have few or none. Many cannabinoid receptors are found in the parts of the brain that influence pleasure, memory, thought, concentration, sensory and time perception, and coordinated movement.

The short-term effects of marijuana can include problems with memory and learning; distorted perception; difficulty in thinking and problem solving; loss of coordination; and increased heart rate.

Research findings for long-term marijuana use indicate some changes in the brain similar to those seen after long-term use of other major drugs of abuse."

Mar. 2004 - National Institute on Drug Abuse (NIDA) 



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.'"

2004 - Charles Ksir, PhD 
Oakley Ray, PhD 



Glen Hanson, DDS, PhD, Associate Director of the National Institute on Drug Abuse (NIDA), stated on July 10, 2003 during a marijuana briefing in Salt Lake City arranged by the White House Office of National Drug Control Policy, as reported in the Deseret News:

"Its [marijuana] effects on the brain function are compounded in adolescents because the behavior center at the frontal cortex is literally not developed...

To use [marijuana] is to take chemical shortcuts to the brain's pleasure center. It is not like riding a roller coaster or jumping out of an airplane. This is adding chemicals to your brain, not inducing a normal sensation. You are changing the way it normally functions and, in effect, creating a mental disorder. The brain bounces and bounces and finally stops bouncing back to normal."

July 10, 2003 - Glen R. Hanson, DDS, PhD 



The Eagle Forum noted in its online brochure "Facts You Need To Know About Marijuana," (accessed Mar. 2, 2006):

"Marijuana is highly deceptive ... Very little of the THC absorbed into the blood of the lungs reaches the brain at the time of the 'high.' Consequently THC appears to be mild even though it is extremely potent.

When pot is smoked regularly, a large supply of THC accumulates in the fat. This produces a high steady level to THC in the blood, which causes continual sedation. The brain is numbed. The mind is in a fog...

Because pot operates so slowly, its damage is often attributed to other drugs the pot smokers use. Nevertheless, medical evidence has proven that marijuana is highly dangerous in and of itself. It seriously harms the brain, the chromosomes, the sex and reproductive organs, the hormones, the lungs, and the immune system."

Mar. 2, 2006 - Eagle Forum 



CON (no)

The Journal of Clinical Investigation stated in its Oct. 13, 2005 article Cannabinoids Promote Embryonic and Adult Hippocampus Neurogenesis and Produce Anxiolytic- and Antidepressant-like Effects," by Xia Zhang et al.:

"We show that 1 month after chronic HU210 [high-potency cannabinoid] treatment, rats display increased newborn neurons [brain cell growth] in the hippocampal dentate gyrus [a portion of the brain] and significantly reduced measures of anxiety- and depression-like behavior.

Thus, cannabinoids appear to be the only illicit drug whose capacity to produce increased hippocampal newborn neurons is positively correlated with its anxiolytic- [anxiety reducing] and antidepressant-like effects."

Oct. 13, 2005 - Journal of Clinical Investigation 



Igor Grant, MD, Executive Vice Chairman at the University of California, San Diego Department of Psychiatry, wrote in his study published in the July issue of the Journal of the International Neuropsychological Society, as reported in a June 27, 2003 Reuters article:

"Smoking marijuana will certainly affect perception, but it does not cause permanent brain damage. 'The findings were kind of a surprise. One might have expected to see more impairment of higher mental function. Other illegal drugs, or even alcohol, can cause brain damage...

If we barely find this tiny effect in long-term heavy users of cannabis, then we are unlikely to see deleterious side effects in indivduals who receive cannabis for a short time in a medical setting...

If it turned out that new studies find that cannabis is helpful in treating some medical conditions, this enables us to see a marginal level of safety."

June 27, 2003 - Igor Grant, MD 



Dale Gieringer, PhD, State Coordinator of CalNORML, wrote in his article "Marijuana Health Mythology," published on the website of the National Organization for the Reform of Marijuana Laws (NORML) (accessed Apr. 20, 2006):

"Government experts now admit that pot doesn't kill brain cells.

This myth came from a handful of animal experiments in which structural changes (not actual cell death, as is often alleged) were observed in brain cells of animals exposed to high doses of pot. Many critics still cite the notorious monkey studies of Dr. Robert G. Heath, which purported to find brain damage in three monkeys that had been heavily dosed with cannabis. This work was never replicated and has since been discredited by a pair of better controlled, much larger monkey studies, one by Dr. William Slikker of the National Center for Toxicological Research [William Slikker et al., 'Chronic Marijuana Smoke Exposure in the Rhesus Monkey,' Fundamental and Applied Toxicology 17: 321-32 (1991)] and the other by Charles Rebert and Gordon Pryor of SRI International [Charles Rebert & Gordon Pryor - 'Chronic Inhalation of Marijuana Smoke and Brain Electrophysiology of Rhesus Monkeys,' International Journal of Psychophysiology V 14, p.144, 1993].

Neither found any evidence of physical alteration in the brains of monkeys exposed to daily doses of pot for up to a year.

Human studies of heavy users in Jamaica and Costa Rica found no evidence of abnormalities in brain physiology.

Even though there is no evidence that pot causes permanent brain damage, users should be aware that persistent deficits in short-term memory have been noted in chronic, heavy marijuana smokers after 6 to 12 weeks of abstinence. It is worth noting that other drugs, including alcohol, are known to cause brain damage."

Apr. 20, 2006 - Dale Gieringer, PhD 



Lynn Zimmer, PhD, Former Professor Emeritus at the Queens College, City University, New York (CUNY), and John P. Morgan, MD, Medical Professor Emeritus at City University of New York Medical School, wrote in their 1997 book Marijuana Myths, Marijuana Facts:

"None of the medical tests currently used to detect brain damage in humans have found harm from marijuana, even from long-term high-dose use....

The claim that marijuana destroys brain cells is based on a speculative report dating back a quarter of a century that has never been supported by any scientific study."

1997 - John P. Morgan, MD 
Lynn Zimmer, PhD