Marijuana’s health effects


In the debate on legalizing marijuana, which Californians will vote on in November in the form of Proposition 19, the health risks of marijuana are often overlooked.

Legalizing marijuana will almost certainly lead to a decrease in its price and an increase in its use, according to a recent Rand Corp. study. And because no drug or medicine is without side effects, increased marijuana use will mean increased health risks.

But what kind of risks? Supporters of legalization say marijuana is no more harmful than caffeine, whereas advocates of criminalization suggest that marijuana is highly toxic. Like other complex health issues, the truth lies somewhere in between.


The California Society of Addiction Medicine is an association of physicians specializing in the treatment of alcoholism and other addictions. Though neutral on the issue of marijuana legalization itself, the society is concerned that Proposition 19 spreads misinformation about marijuana by stating: “Cannabis is not physically addictive, [and] does not have long-term toxic effects.” Unfortunately, this statement ignores a large body of national and international scientific research over the last four decades that proves marijuana can be addictive.

About 9% of adults who use marijuana develop an addiction to it. Among people who begin smoking before the age of 18, this number is as high as 17%. Although addiction to marijuana does not cause dramatic physical dependence, it can lead to substantial problems in education, work and relationships. In fact, addiction to marijuana is defined by the inability to stop using despite recognition of harmful consequences. Without harmful consequences, there is no diagnosis of addiction.

The short-term effects of marijuana intoxication are well established. As part of the high produced by marijuana, intoxication impairs memory and learning. Marijuana use also impairs driving, causing a twofold to threefold increase in accidents. Though not as dramatic as the fifteenfold increase in accidents caused by alcohol intoxication, marijuana’s impact on traffic safety does have significance.

The long-term effects of marijuana are not often recognized because they are subtle, but they can have a cumulative impact over time. In people with preexisting vulnerabilities, marijuana use can unmask psychiatric problems such as schizophrenia. Many people with anxiety and depression use marijuana to soothe their symptoms; however, there is evidence that over time it may actually make these problems worse.

Smoked marijuana irritates the linings of the respiratory passages and can lead to inflammation and bronchitis. Although marijuana has not been definitively shown to cause cancer, smoked marijuana has been linked to precancerous changes in the lungs.

These long-term effects of marijuana are not as dramatic as those seen in other, “harder” drugs of abuse, but they do take a toll, and that toll appears to be greatest among people who begin smoking marijuana during adolescence, before the brain and body are finished maturing.


Long-term marijuana use has not been shown to reduce general measurements of intelligence; however, there is evidence that the processing of highly complex information is slowed. Even after 28 days of abstinence, brain scans of long-term marijuana users show less activity in regions serving memory and learning.

The bottom line is that although marijuana is less hazardous to health than many other legal or illegal drugs, it is not without risk, and voters and potential users need to fully understand this.

Among treatment professionals, there is widespread consensus that incarcerating people for marijuana use or possession is wrong, and therefore, an alternative policy is long overdue. If we are to promote the public health, however, we need to anticipate and deal with the consequences of our policy decisions. California already does an abysmal job of making treatment resources available to people addicted to marijuana and other drugs, including adolescents. In the advent of legalization, there will be an increased need for prevention programs, education and treatment services.

The decision to legalize marijuana is in the hands of California voters. It is vital that they make their decision on the basis of accurate facts. And if Californians do legalize marijuana, they should insist that revenue from its taxation be applied toward any problems that arise from its increased use.

Itai Danovitch is an assistant professor of psychiatry and behavioral neurosciences at Cedars-Sinai Medical Center. He is a member of the California Society of Addiction Medicine’s Committee on Public Policy.