How Safe is Marijuana? A Toxicological Perspective

by David on March 25, 2013

I’ve been contemplating marijuana since Colorado and Washington passed ballot initiatives legalizing the stuff back in November.  Not as a user, mind you – but as a toxicologist entertaining the idea of a new recreational drug entering the fray currently ruled by alcohol and tobacco. What are the health hazards (or benefits?) associated with smoking a joint? Or five? Is cannabis really the nightmare drug that my elementary school D.A.R.E. program led me to believe? (Hint: it isn’t.) Or is it merely a misunderstood medicine? Fortunately, a handful of groovy scientists have been digging on the problem in recent years, and their findings may surprise you. 

I’m not sure which is worse: the fact that I’ve had this shirt for fifteen years, or the fact that it still fits. 

What’s the Dose? How Much Will Kill You?

The LD50 is one of the more recognizable classical toxicological data points, and it’s a good place to start our investigation: what is the Lethal Dose for 50% of a test population? delta 9-Tetrahydrocannabinol (THC) is the primary active compound in marijuana, and it’s LD50 in rodent studies (a general starting point for toxicological research) is absurdly high, as is the human LD50, which is theoretically between 15 and 70 grams. To put that in perspective, the casual user (once a month or so) generally only needs about 2-3 mg of THC to become intoxicated, while habitual users might need between five and ten times that amount. Since 3 mg = 0.003 g, a casual user would need to smoke about 5000 times their normal amount to approach a potentially lethal dose.

No word yet on the lethal dose of ice cream. Can we just assume there isn’t one?

Acute Effects  

Okay, so it probably won’t kill you. But what kind of negative acute effects are we dealing with? What bad stuff can happen upon first exposure? The most common negative acute side effects are panic, tachycardia (rapid heartbeat), impaired information processing and reaction time, and decreased motor coordination. In rare cases, marijuana use can precipitate or exacerbate latent schizophrenia, but I’ll get into that later.

The Big 3: Cancer, Developmental, and Reproductive Effects

One of the most important distinctions made in the toxicological assessment of chemicals is that of carcinogenic versus non-carcinogenic compounds, and for the moment, marijuana falls unsteadily into the latter category. There is a good chance that all that smoke inhalation is causing cancer of some sort, but the epidemiological studies have had a heck of a time trying to tease apart the confounding effect of cigarette smoking, since many people who use marijuana also smoke cigarettes, which are known to cause a variety of cancers.

Another important distinction is that of teratogenicity: does the compound cause birth defects? Again, at the moment, no, there isn’t any evidence that marijuana causes birth defects, although many researchers are quick to warn that there isn’t a tremendous amount of data on the subject yet.

Things aren’t looking too good on the reproductive health front, however. Emerging data suggests that chronic marijuana use can reduce fertility in women, and there evidence that THC can effect sperm count and motility.

“I was thinking about it, man, and like, why even bother to swim upstream?” “I feel you, salmon-bro.”

Neurological and Psychological Effects

For reasons that aren’t yet clear, there is a small chance that smoking marijuana will precipitate schizophrenia. But before you get to work on that Nancy Reagan shrine, there are a few things worth noting about this unusual phenomenon. For starters, there is a probable genetic component to this, and individuals that don’t have a family history of schizophrenia are extremely unlikely to develop it as a result of marijuana use. Secondly, only about 3% of heavy users are at risk for developing schizophrenia, and if the disease doesn’t manifest during puberty, it’s not likely to during adulthood. As Bostwick (2012) notes, moderate marijuana usage after the age of 18 is not associated with increased rates of mental illness.

In addition to possible schizophrenia, there are several other neuropsychological effects that were observed among adolescents that abused marijuana heavily, including: memory deficits, reduced information processing speed, susceptibility to mood and anxiety disorders, and greater likelihood of dependence. Much like alcohol and cigarettes, use and abuse of marijuana before and during puberty can have serious consequences for the developing brain and should be avoided at all costs.

(source: alright, fine, I agree with you: kids shouldn’t smoke pot.

Hooked on Hashish: Marijuana Dependency

Contrary to what many would like to believe, marijuana can be addictive, and between 9 and 11 percent of those who toke will get hooked. However, this statistic only holds out for individuals who start using marijuana during their teen years and early 20s – the percentage of first-time users over the age of 25 who become dependent is practically zero. Also, a 1 in 10 addiction rate sounds bad until you consider the rates of dependence for first time users of alcohol (15%), opioid (23%), cocaine (17%), or nicotine (32%).

And the Rest: Other Chronic Effects

Chronic use (and to be clear, I’m talking about several joints a day, every day, for 10 years) of marijuana is associated with many of the same respiratory diseases that plague tobacco smokers, including emphysema and bronchitis. Since marijuana cigarettes tend to be unfiltered and the smoke is held in the lungs for longer, a lot more tar and nasty gunk is inhaled per drag. An increase in respiratory infections has also been noted among chronic users, as marijuana suppresses the immune system. Memory problems and other cognitive issues may develop over time, and the potential for cardiovascular complications increases as well.

Reefer? Pah, you’ll never see me TOUCH the stuff. It’s unfiltered Lucky Strikes for me, yessir.

The Thin Green Line

So to answer that original question – how safe is Marijuana? – we may need to start by saying “compared to what?” Taken together, the aforementioned data suggests that marijuana isn’t deadly, but that it does pose a non-trivial hazard to young persons and individuals with a genetic predisposition for schizophrenia, and that chronic use of marijuana may result in fertility problems, cardiovascular disease, and respiratory distress.

How’s that stack up to alcohol and cigarettes?

The WHO has stated that globally, tobacco is the single greatest cause of preventable death, lopping about 17 years off the lives of habitual smokers – to say nothing of the effects of secondhand smoke. Alcohol is associated with dozens of cancers, and liver and heart diseases. And both cigarettes and alcohol have dire consequences for embryonic development. Interestingly, there has been an ongoing debate about the role of cigarettes and the development of schizophrenia, some researchers finding it palliative, others finding it causative, and yet more finding it merely related.

So, Is Marijuana Safe? Well, from where I’m sitting, it’s no more or less safe than alcohol, cigarettes, or prescription opiates for that matter. Everything is toxic at some dose. The key, as always, is moderation and responsible use – and definitely no use among minors. And who knows, maybe if we can overcome our social vendetta against marijuana, we might uncover some medical benefits? Alas, that is a topic for another post.

Pictured: the undisclosed location where the author intends to hide from vitriolic comments.