Deprecated: mysql_connect(): The mysql extension is deprecated and will be removed in the future: use mysqli or PDO instead in /home/content/28/3595028/html/connect.php on line 2
You are in Research


from California NORML Reports, April 1992


Two new scientific studies have failed to find evidence of brain damage in monkeys exposed to marijuana, undercutting claims that marijuana causes brain damage in humans.

The studies were conducted by two independent research groups. The first, conducted by Dr. William Slikker, Jr. and others at the National Center for Toxicological Research in Arkansas examined some 64 rhesus monkeys, half of which were exposed to daily or weekly doses of marijuana smoke for a year. The other, by Gordon T. Pryor and Charles Rebert at SRI International in Menlo Park, California, which is still unpublished, looked at over 30 rhesus monkeys that had inhaled marijuana one to three times a day over periods of 6 to 12 months. Neither study found evidence of structural or neurochemical changes in the brains of the monkeys when examined a few months after cessation of smoking.

The new results cast doubt on earlier studies purporting to show brain damage in animals. The most famous of these was a study by Dr. Robert Heath, who claimed to find brain damage in three monkeys heavily exposed to cannabis. Heath's results failed to win general acceptance in the scientific community because of the small number of subjects, questionable controls, and heavy doses.

Subsequent rat experiments by Dr. Slikker and others reported persistent structural changes in the brain cells of rats chronically exposed to THC. The studies did not show that pot kills brain cells, as alleged by some pot critics, but they did show degeneration of the nerve connections between brain cells in the hippocampus, where THC is known to be active.

Although scientists have regarded the animal evidence as inconclusive, some critics have cited it as proof that pot causes brain damage in humans. Thus Andrew Mecca, the director of California Department of Alcohol and Drug Abuse, recently stated on the Ron Reagan, Jr. talk show (Sep. 2, 1991) that marijuana "leaves a black protein substance in the synaptic cleft" of brain cells, a claim apparently based on Heath's monkeys. When asked by a NORML member for his evidence, Mecca sent a list of three references, none of which turned out to have anything to do with brain damage.


A new study of children born to marijuana-smoking mothers found no link between marijuana exposure and the birth defects of fetal alcohol syndrome (FAS). The new study, by Dr. Susan J. Astley of the University of Washington, published in the January, 1992 issue of Pediatrics, contradicted a 1982 study by Dr. Ralph Hingson, in which prenatal exposure to marijuana was found to increase the risk of FAS.

Hingson's results, which have not been replicated, have been questioned on various methodological grounds, in particular the difficulty of controlling for combined drinking and pot use.

The new study looked for facial deformities symptomatic of FAS in 40 children whose mothers had smoked marijuana heavily during pregnancy and 40 controls, It found no association between marijuana and FAS, but deformities were observed in children of women who drank 2 ounces of alcohol per day or took cocaine.


A new study by Dr. Robert Block at the University of Iowa disputes the commonly held notion that marijuana alters the level of testosterone and other sex hormones.

The study contradicted a widely publicized 1974 study by Dr. R.C. Kolodony, which reported decreased testosterone levels in men who smoked marijuana chronically.

The U. of Iowa study found that chronic marijuana use had no effect on testosterone, luteinizing hormone, follicle stimulating hormone, prolactin and cortisol in men or women.

Noting that six other studies had failed to show lowered testosterone levels in men, Dr. Block concluded: "It appears that marijuana, even heavy use of the kind that's typical in the United States, doesn't alter testosterone levels."

ukcia banner
Use this banner to link to UKCIA

Page designed and maintained by UKCIA