Is Weed Bad For You? The Arguments For and Against Marijuana's Supposed Health Risks

According to a new report this week from a United Nations health commission, marijuana consumption has increased among Americans over the past four years, and more people are seeking treatment for marijuana-related health concerns. The number of people over the age of 11 who’ve used marijuana at least once in the last year rose from 10.3 percent to 12.1 percent in 2012, while there was a 59 percent increase in cannabis-related hospital visits from 2006 and 2010. At the same time, public perception of marijuana-related health risks has declined, while THC levels in marijuana have increased. While the measurements were taken before Colorado and Washington legalized recreational marijuana use, the report is sure to factor into the ongoing national debate on marijuana legalization.

We thought now would be a good time to take a look at some common marijuana-related health claims and see how much water they actually hold. We'll give you both sides of the argument so you can decide for yourself.

Claim: Marijuana harms the lungs

Argument It Does: Marijuana requires smokers to take a longer and deeper inhalation than tobacco in order to feel its effects, and burns at a higher temperature than tobacco. As such, on a “per puff” basis, marijuana smoke is more harmful to the lungs than cigarettes: It contains about five times the concentration of carbon monoxide and three times as much tar. In addition, about one-third the tar is retained in the respiratory tract after smoking marijuana when compared with tobacco.

Marijuana smoke also contains higher concentrations of certain carcinogens, notably benzopyrene and benzathracene, than cigarettes, and lung biopsies from marijuana-only smokers have shown tissue damage that’s recognized to be a precursor to cancer.

Argument It Doesn't: Despite the carcinogens, tar content, and tissue damage that marijuana causes, no study has found that marijuana use actually leads to an increased risk of lung cancer.

To take a recent example, a 2013 study from the University of California at Los Angeles concluded that “habitual use of marijuana alone does not appear to lead to significant abnormalities in lung function,” and furthermore, “findings from a limited number of well-designed epidemiological studies do not suggest an increased risk for the development of either lung or upper airway cancer from light or moderate use.” This confirmed the findings of a 2004 study from Washington State and 2006 study published by the American Association for Cancer Research.

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Of course, when baked and ingested orally, marijuana has no effect whatsoever on the lungs.

Claim: Marijuana has an adverse effect on brain development

Argument It Does: Numerous studies have concluded that early marijuana use, particularly during the teenage years, has measurable effects both on brain structure and cognitive performance. In a 2013 study published in Oxford University Press, researchers observed “cannabis-related shape differences” in numerous parts of the brain in marijuana smokers, and “subcortical neuroanatomical differences” between those who smoke and those who don’t. It also found that people who smoke marijuana have poorer working memory, a conclusion that’s been backed up by other studies.

In addition, people who begin smoking marijuana during their late teens have been found to have shorter attention spans, poorer visual search abilities, and worse abstract reasoning skills. One study also found that people who start smoking marijuana as teens have lower IQs as adults — and even when they stop smoking, their IQs don’t increase.

Argument It Doesn't: It’s very difficult to determine whether the relationship between brain functioning and marijuana use is actually causal or merely correlative. The problem is confounding factors — especially socioeconomic status, which is correlated both with poorer cognitive functioning and early exposure to marijuana. Researchers in Norway concluded in 2013 that most, if not all, of the correlation between neuropsychological function and marijuana use can potentially be chalked up differences in socioeconomic status between smokers and non-smokers.

Education is another confounding factor. Mandatory schooling gives people a boost in their IQ, but that boost slowly disappears as schooling ends. As a result, people who don’t go to college often show decreases in IQ during their 20s — and such people are also more likely to be marijuana smokers.

Claim: Marijuana is addictive

Argument It Is: Strictly speaking, marijuana has the potential to be addictive. Around nine percent of people who smoke marijuana develop a dependency that causes problems in their work and relationships; that number increases to 17 percent for smokers who start young, and 25-50 percent for people who smoke it every day.

Argument It Isn't: An addiction rate of nine percent is amongst the lowest of any commonly-used drug in the country. It’s less than tobacco (32 percent), heroin (23 percent), cocaine (17 percent) and alcohol (15 percent). Over half of all Americans report having used marijuana by their 20s, compared with two percent for heroin, and yet heroin users account for almost as large a share of addicts in treatment (14 percent) as do marijuana users (16 percent).

While most addiction specialists reject the dichotomy of “physical vs. psychological” addiction, the potential for physical harm can’t be entirely ignored in conversations about addiction. For example, one of the big reasons heroin and cocaine addictions are so dangerous is the potential for overdose, which can be fatal. Nobody has ever once died from a marijuana overdose. Alcohol withdrawal, meanwhile, can literally kill you, while marijuana withdrawal, in the words of one expert, “is annoying, but isn’t life-threatening.”

Claim: Marijuana is a gateway drug

Argument It Is: A 2012 Yale study found that, amongst men and women between 18 and 25, those who’d previously used marijuana were more than twice as likely to have prescription drug abuse problems than those who were always pot-free.

Argument It Isn't: That Yale study also found that alcohol and tobacco use were substantially more predictive of prescription drug abuse than marijuana: While 34 percent of those with prescription drug problems had used marijuana, 56 percent had started by smoking cigarettes, while 57 got their start with alcohol. So, if those statistics are proof of a gateway effect, cigarettes and alcohol are bigger gateway drugs than marijuana.

But these statistics aren’t proof of a gateway effect, because once again, correlation isn’t causation. In fact, researchers have consistently found that there isn’t a causal effect. In 1999, Congress commissioned a study on marijuana’s status as a gateway drug; the commission found “no conclusive evidence that the drug effects of marijuana are causally linked to the subsequent abuse of other illicit drugs,” and concluded that “most drug users begin with alcohol and nicotine before marijuana.” This finding was confirmed in 2003, and again in 2006.

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