If you are considering smoking pot—or quitting—here is what you need to know. Smoking marijuana once is very unlikely to harm you. It takes at least 15 grams of cannabis to kill a person, and probably much more than that. A healthy person would have to smoke dozens of joints in a single session to risk death from overdose. People who do die from the acute effects of marijuana die in accidents: A recent study suggested that more than 10 percent of drivers killed in car accidents test positive for cannabis.*
The more likely risk from trying marijuana is dependence. There is a debate over whether marijuana is addictive, but you needn’t bother with it—it’s mainly about semantics. The fact is, approximately 9 percent of people who start smoking pot become dependent by ordinary medical standards. That’s low compared with dependence rates for other drugs: More than 15 percent of people who drink become alcoholics, and 32 percent of people who try cigarettes get hooked. Still, you should think seriously about a 1-in-11 chance of addiction, especially if you have a personal or family history of substance abuse.
What sorts of health risks are these regular cannabis users taking? It’s extremely challenging to study the long-term health effects of marijuana in humans. You can’t legally ask 1,000 people to smoke three joints a week for 40 years just to see what happens, so researchers can only compare health data from people who admit to smoking pot with data from people who don’t admit to it. Retrospective correlational studies like these raise all kinds of problems, such as matching the groups for confounding variables. (Do they smoke cigarettes? Do they have a family history of cancer? What do they eat? Do they exercise?) Even if you assume that everyone is telling the truth, there are also bound to be wide variations in how much pot the subjects used. Most studies suggest that any potential health risks of cannabis are dose-dependent—people who smoke only a little face very few health risks, while people who smoke a lot are more likely to get sick—but this is still largely a matter of conjecture.
With that caveat out of the way, here are some findings. Studies consistently show that frequent marijuana smoking is associated with some forms of respiratory dysfunction. Smokers report problems with coughing, wheezing, and phlegm. Lung cancer is a murkier issue. Cannabis smoke contains higher concentrations of some carcinogens than cigarette smoke does. Some large studies show increased prevalence of respiratory tract cancers in cannabis users, while others find no correlation.
With the legalization of recreational marijuana in some states, many people have asked whether they can minimize cancer risk by ingesting rather than smoking cannabis. It’s a reasonable suggestion. At this point, however, the question is unanswerable. There simply aren’t enough people with a long history of eating marijuana, but not smoking it, to put together a study. Ingestion may be risky, because it seems easier to overindulge in food products than in smoking. Colorado is currently reviewing its regulations after accidental deaths involving ingested cannabis. Keeping cannabis brownies is especially risky if you ever have children in your home. A study released last year suggested that an increasing number of children in Colorado are accidentally eating marijuana-laced food products.
The cognitive effects of chronic marijuana use are uncertain. If you’re an adult who smokes occasionally, there appears to be little or no reason to believe your mental performance will suffer. Several studies also show that those who experience impairments may recover if they stop smoking. Heavy, long-term smokers may experience memory and attention loss. There is also some indication that heavy marijuana users are more likely to be diagnosed with schizophrenia, but it’s not clear which is the cause and which the effect, if indeed there is such a relationship; it may be the case that people with schizophrenia are self-medicating with marijuana.
As with alcohol and tobacco, it’s fairly clear that minors should not use marijuana. Many studies show that kids who smoke pot do poorly in school, and some studies suggest that they commit suicide at higher rates. Although the causal relationship isn’t clear, the risks are too great.
You probably have plenty of other questions. For example, is marijuana less bad for you than alcohol or tobacco? The comparison is basically impossible to make. Mountains of data link cigarette smoking to a staggering collection of adverse events. It’s difficult to know whether the same goes for marijuana, because fewer people smoke it, and those who do typically smoke less pot than cigarette smokers do tobacco. Comparing alcohol with marijuana—aside from differences in acute toxicity and driving competence—is also impractical.
Seriously, though, if you’re trying to decide among smoking pot, taking up cigarettes, and drinking alcohol based on health risks, I suggest finding a different hobby.
*Correction, May 1, 2014: Due to an editing error, this piece misstated that almost 25 percent of drivers killed in car accidents test positive for cannabis. Almost 25 percent test positive for non-alcohol drugs; of those, about 12 percent test positive for cannabis. (Return.)