A still from "Reefer Madness," an anti-marijuana propaganda film from 1936.

You may have read this week that a new "20-year research study" on marijuana use "finally demolishes claims that smoking marijuana is harmless," and has found that it "makes you stupid," that "smoking marijuana over the long-term can develop cancer" [SIC], and that marijuana is "as addictive as heroin." At least, that's what you'd conclude if you'd read most media coverage of the study. But if you'd actually read the study yourself (which I highly recommend!), you'd likely walk away with very different conclusions.

The paper in question is a review of 20 years of existing research into the health effects of marijuana use. By design, it doesn't tell us anything we don't already know. The focus is almost exclusively on the effects of long-term heavy (daily or near-daily) marijuana use: "this paper deals with the adverse effects of cannabis smoking, especially the adverse health effects of regular, typically daily, cannabis smoking," the author, Wayne Hall, a drug adviser to the World Health Organization, writes.

Setting aside the alarmist accounts of the study in many media outlets, here's what Hall actually found:

You can't OD from marijuana
"The estimated fatal dose [of THC, the primary active compound in marijuana] in humans derived from animal studies is between 15 and 70 grams. This is a far greater amount of cannabis that even a very heavy cannabis user could use in a day," Hall writes. The average joint contains about a half a gram of marijuana, and the average potency of seized marijuana in 2013 was 12.58 percent, which means there are about 0.06 grams of THC in the average joint, which means that somebody would need to smoke somewhere between 238 and 1,113 joints in a day - or at least 10 joints an hour, for 24 hours straight - before overdose could become a realistic concern.

Don't drive stoned

Stoned driving is considerably safer than drunk driving, but it's still a dumb thing to do, Hall writes. "Cannabis users who drive while intoxicated increase their risk of motor vehicle crashes 2–3 times as against 6–15 times for comparable intoxicating doses of alcohol."

Don't smoke weed while you're pregnant

Studies have demonstrated a link between marijuana use during pregnancy and low birth weight, "although the effect was smaller than that for tobacco smoking," Hill notes. There is some evidence of a link between prenatal cannabis exposure and a variety of problems later in life, like lower IQ and behavioral problems. But, "uncertainty remains because of the small number of studies, the small samples of women in each and the researchers’ limited ability to control for the confounding effects of other drug use during pregnancy, maternal drug use post-birth and poor parenting."

In short, "it is prudent to counsel women against using cannabis during pregnancy," Hall concludes.

You can get addicted to marijuana, but it's highly unlikely

People who try marijuana are significantly less likely to become dependent on it than users of just about any other drug, including tobacco, heroin, cocaine, alcohol or stimulants: "The life-time risk of developing dependence among those who have ever used cannabis was estimated at 9% in the United States in the early 1990s as against 32% for nicotine, 23% for heroin, 17% for cocaine, 15% for alcohol and 11% for stimulants." More than nine-in-ten people who try marijuana don't get addicted to it.

The risk of addiction is higher (one-in-six) if you start using in your teens. Dependent users can experience withdrawal symptoms when they quit, including "anxiety, insomnia, appetite disturbance and depression." That said, "The adverse health and social consequences of cannabis use reported by cannabis users who seek treatment for dependence appear to be less severe than those reported by alcohol and opioid-dependent people," Hall writes.

There's some evidence that heavy marijuana use can lead to cognitive impairment, particularly if you start in your teens

Some studies have found deficits in learning and attention among regular marijuana users, but "it still remains unclear whether cognitive function recovers fully after cessation of longterm cannabis use." One particularly well-known study posited a link between IQ declines and teen marijuana use, but other research has contested that claim.

There may be a link between heavy teen marijuana use and lower educational attainment, but again the causality is far from clear

A recent study concluded that heavy marijuana use in your teens decreases your likelihood of graduating high school, but as I noted at the time, there could be any number of confounding factors contributing to this finding. Hall notes that "An analysis of twins who were discordant for early cannabis use [e.g., one used while the other didn't] found no difference in risk of early school-leaving between the twins who did and did not use cannabis, suggesting that the association was explained by shared genetic and environmental risk factors. These findings are supported by two earlier analyses of US twin-study data."

There may be links between teen marijuana use and the use of harder drugs, but this research is hotly contested

Studies show some evidence for the so-called "gateway theory," but according to Hall "we don't know whether the link is causal." Many researchers suspect that to the extent there is a gateway drug, it's tobacco, not marijuana.

There is some evidence that marijuana use doubles the likelihood of developing a psychosis

But, "it is difficult to decide whether cannabis use has had any effects on psychosis incidence, because even if the relationship were causal, cannabis use would produce a very modest increase in [overall] incidence," Hall writes. It nevertheless seems prudent to suggest that if you have pre-existing mental conditions and decide to use marijuana (or alcohol or any other drug, for that matter), you're rolling the dice when it comes to your mental health.

There's no good evidence of a link between marijuana use and depression, but some evidence of a link with suicide.

Some studies have suggested a weak link between marijuana use and depression, but those links disappear after you adjust for confounding factors. There seems to be better evidence for connections between marijuana and increased risk of suicide, but several analyses of these studies suggest "that these studies and measures used were too varied to quantify risk meaningfully, and most of the studies had not excluded reverse causation or controlled adequately for confounding," Hall writes.

Regular marijuana use is linked to increased risk of chronic bronchitis

Yes, inhaling flaming plant material into your lungs on a regular basis could impair your respiratory function.

Marijuana smoking probably increases the risk of heart attack in middle aged adults

Among middle-aged and older adults, there is evidence of increased risk of heart attack in the hour or so after smoking marijuana. There is also limited evidence that smoking marijuana may provoke cardiovascular events in young people with undiagnosed cardiovascular disease.

There may be a link between marijuana use and testicular cancer in men

"Studies found a doubling of risk of non-seminoma testicular tumours among cannabis users and suggestive evidence that risk increased with earlier initiation and more frequent use of cannabis," Hall writes. "It is also a biologically plausible effect, given that cannabinoid receptors are found in the male reproductive system."


The overall take-home message from Hall can be summed up as follows: if you use marijuana, don't overdo it. If you're a teen or a pregnant lady, best not to use marijuana at all.

It's important to note that any effects noted above are primarily among heavy, long-term users. Long-term use of any drug - be it marijuana, alcohol or nicotine - will come with negative health consequences, even if, according to this paper, the consequences are not as bad with marijuana use.