Dear Dr. Roach: I am absolutely convinced that marijuana is a serious health hazard. As the legalization of marijuana appears to be spreading, are you aware of any authoritative medical studies confirming the health hazards of marijuana and that it is a gateway drug to more-serious drug addictions?
This is an extremely large and controversial topic that I can't do justice to in a single newspaper column, but I can at least outline some of the issues.
I absolutely can confirm that cannabis has health hazards. Cannabis use disorder is one: It is diagnosed when a person uses cannabis in a problematic way, leading to significant impairment or distress. Commonly, this involves using cannabis more often than a person wants to and results in problems at school, work or home. Cannabis use disorder is similar, in this way, to alcohol use disorder. However, most people who use cannabis, like those who use alcohol, do so without developing cannabis use disorder.
There are other well-described risks. Cannabis adversely affects driving ability and may cause long-term problems in verbal memory (the data are conflicting, and a large study is underway). Smoking cannabis is damaging to the lungs (significantly less so than tobacco, though, since the amount smoked by most cannabis users is smaller). There may be an increased risk of testicular cancer with regular use in men, and there is a syndrome called “cannabinoid hyperemesis syndrome,” causing uncontrollable vomiting. Teens who use cannabis have an increased risk of schizophrenia. However, deaths from acute cannabis use are almost unheard of, in sharp contrast with alcohol or opiates, and the effect on driving, when it comes to causing a collision, is less with cannabis than it is with alcohol.
As far as whether cannabis is a “gateway” drug, the evidence suggests that people who will go on to use other, more harmful sources often do start with cannabis and there may be changes in a user’s brain chemistry making this more likely. However, the majority of people who use cannabis do not go on to use other substances. It is also possible, perhaps likely, that people who are at high risk for using opiates, cocaine or other drugs will start with what is available, and tobacco, alcohol and cannabis are the most available substances. Cannabis is almost certainly safer than both tobacco and alcohol.
Finally, one needs to consider that people who use cannabis may be less likely to have more-harmful habits, such as overuse of alcohol. Early data from areas that have legalized recreational marijuana suggest reduced rates of motor vehicle accidents, for example.
I receive many letters from people whose opinion is that cannabis is a miracle substance that will cure everything, but this is clearly not the case. Still, medical cannabis is finding some legitimate use, such as with some types of childhood epilepsy. Cannabis extracts, THC and CBD, are likely to have other medical benefits. Recreational cannabis use has the potential for harm, but may actually cause a net reduction in mortality by reducing problem drinking.
I cannot overemphasize the need for better studies to determine best strategies to reduce the problem use of any recreational substance, legal or not. A scientific review is available at the National Institute on Drug Abuse (tinyurl.com/NIDA-cannabis).
Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu.