A 2017 study published in the British Medical Journal, which was conducted by the University of Oxford and the University College of London, found that even moderate alcohol consumption could cause damage to the brain. The study looked at 500 adults over the course of 30 years and tracked their weekly alcohol consumption.
The adults in the study consumed various amounts of alcohol, the levels of which were self reported in the form of units per week. A unit of alcohol in the study was considered 25 ml liquor, 250 ml beer or 76 ml of wine. Even people in the study who consumed 14 to 21 units of alcohol per week, which is well within the guise of recommended limits of moderate alcohol consumption determined by health services, were three times more likely to suffer from hippocampal atrophy than people who consumed lower amounts of alcohol.
Heavy drinkers in the study, who consumed 30 units of alcohol a week, were most at risk for brain damage, followed by moderate drinkers, and then by minimal to light drinkers, who were the least at risk for brain damage. Though this study was observational, and all of the alcohol consumption levels were self-reported, the study points to the fact that alcohol consumption, even in moderate amounts, could cause brain damage.
It's important to note, however, most of the people in the study were white, middle-aged males. Alcohol is normally left out of discussions about what drugs can do to the brain and what effects it can have on long-term health. It's important to recognize, however, that despite various folks claiming cannabis may have major adverse effects on brain health, alcohol may be more likely to blame.
Another 2017 study conducted by the University of Colorado at Boulder and the Oregon Health and Science University compared the effects of cannabis and alcohol on the brain. The study, which was published in the journal Addictions, found that alcohol created adverse structure changes in the brain, while cannabis didn't. The study used neuro-imaging data to look at changes in the brain structure in almost 1,300 adolescents and adults who consumed alcohol and/or cannabis. No brain changes were seen in the neuro-imaging of patients who consumed cannabis. Though the study isn't conclusive, it's consistent with previous findings that cannabis doesn't cause damage to the brain.
A 2016 study did show some changes in grey matter in the brain connected to cannabis consumption, though it was only associated with consumption of cannabis by adolescents before adulthood. Grey matter is responsible for large amounts of information processing in the brain, and large amounts of grey matter are associated with higher levels of intelligence. The study found that there was a decrease in grey matter associated with cannabis consumption only in adolescence, but young adults who weren't smokers before adulthood, no matter how extensive their consumption of cannabis was, didn't show a decrease in grey matter.
Though the studies are preliminary, they do indicate that cannabis is far less likely to cause brain damage compared to alcohol. Though alcohol is considered to be less dangerous, it actually has the potential to cause far more damage to the human brain than cannabis. The potential for brain damage is often a rallying cry by anti-cannabis advocates to justify the continued prohibition of a potentially beneficial medication, but scientific studies have proved that these claims are completely unfounded.