One Statistic Debunks One of Marijuana Legalization Opponents' Favorite Myths

How much weed does it take to kill you? As it turns out, it's probably easier to accidentally overdose on water than on ganja.

The LD50 (the dose required to kill half of the subjects in a test population) of marijuana's active ingredient THC is in the neighborhood of 50-70 grams, which is virtually impossible for any consumer to achieve in a normal setting. Casual users need just 2-3 mg of THC to become intoxicated, and habitual tokers might need somewhere in the neighborhood of five to 10 times that amount. The University of Michigan's Mind the Science Gap estimates this means someone would need to smoke around 5,000 times their average consumption to die from THC poisoning.

Water, on the other hand, has a LD50 of 90 grams per kilogram of body weight. While that's higher than marijuana's, it practical terms it's actually much easier to OD on water in the course of your daily life. In 2007, a California woman perished from water intoxication after drinking six liters (about 25 glasses) of water over the course of three hours in an attempt to win radio show water-drinking contest.

Scientific American's Coco Ballantyne wrote that water intoxication has caused deaths from hazing rituals and Ecstasy-popping club-goers who over-hydrate, and noted that potentially dangerous levels of hyponatremia have been noted in nearly one-sixth of marathon runners.

"Thus, water's body count," writes RealClearScience's Ross Pomeroy, "remains higher."

But haven't people overdosed on pot already? In February, the media lit up over the story of the deaths of two German men who appeared to have cardiac arrhythmia triggered by marijuana. But as Pomeroy notes, these deaths weren't from overdoses:

The researchers who reviewed their deaths in the journal Forensic Science International reported that "the younger man had a serious undetected heart problem and the older one had a history of alcohol, amphetamine and cocaine abuse." Since all other causes of death were ruled out, the researchers assumed that marijuana spiked their heart rates and blood pressures, causing their hearts to fall out of rhythm.

Moreover, these two deaths have to be put in the context of other substances that can raise the risk of cardiac arrest, like prescription painkillers or testosterone. 

Safe weed? Compared to poisons like cyanide or strychnine, weed is pretty much harmless. Compared to other drugs like meth, heroin or cocaine based purely on their ability to straight-up kill you, marijuana might as well be milk. And it could be realistically argued that marijuana is less deadly than water.

Even drug warriors admit the risk of lethally overdosing on marijuana is negligible. DEA administrative judge Francis Young ruled in 1988 that "A smoker would theoretically have to consume nearly 1,500 pounds of marijuana within about 15 minutes to induce a lethal response." In other words, you'd die of smoke inhalation and/or oxygen deprivation well before you reached the lethal dose.

This information will probably disappoint opponents of marijuana legalization fixated on the "deadly effects" of the drug. Dr. Alan Shackelford told the Denver Post, "There's been no history of any verified reports of a death from cannabis ever. Cannabis can cause an increased heart rate, and there's a possibility that it could cause a problem with someone with a pre-existing heart disease — for example, somebody with an elevated heart rate. But there's no known dose of cannabis that could kill a human."

There are still risks. None of this is to say that consistent marijuana use can't have detrimental effects. Like any other substance (say, alcohol), moderation is key. Smoking or eating too much weed can be problematic. Users have often felt like they were dying, triggering instense panic attacks. In some cases, particularly involving edible THC products, over-indulging in marijuana has been linked to injury or death. And combining marijuana with other substances can have deadly results; driving high is already ill-advised, and the risk of being involved in a deadly alcohol-related car crash nearly doubles from 13 times to 24 times when weed is added to the mix.

But a marijuana overdose simply doesn't exist in any meaningful pharmacological sense. It is more or less impossible to die from THC alone, making it a remarkably safe substance compared to virtually all of the other recreational drugs (including alcohol). Any argument stating otherwise is just wrong.