There's a very common drug-policy talking point that's meant to convey the absurdity of the war on drugs: Alcohol is more dangerous than marijuana, even though alcohol is legal and marijuana is not.

Perhaps the biggest supporting evidence for this point is a 2010 study published in The Lancet that ranked alcohol as the most dangerous drug in the United Kingdom, surpassing heroin, crack cocaine, and marijuana. That study has drawn widespread media attention, appearing in outlets like the Washington Post, the Guardian, the New Republic, and here at Vox.

Although drug policy experts generally don't dispute the assertion that alcohol is more dangerous than pot, the study, led by British researcher David Nutt, is quite controversial. Experts see the rankings as deeply flawed, largely because they present the harms that come from drugs in a rather crude, one-dimensional manner. Even Nutt has acknowledged that the study is imperfect.

This may seem like a petty academic squabble, but it's quite important as researchers and lawmakers try to advance more scientific approaches to drug policy. Finding the best method to evaluate the risks of drugs is much more complicated than assigning numeric rankings.

What the UK analysis does

Nutt's analysis measures two different issues related to drug use in the UK: the risk to an individual, and the damage to society as a whole.

The individual scores account for a host of variables, including mortality, dependence, drug-related family adversities, environmental damage, and effect on crime.

Even if two drugs score similarly in Nutt's analysis, the underlying variables behind the scores can be completely different. For instance, heroin and crack cocaine are fairly close in the rankings. But heroin scores much higher for mortality risk, while crack poses a much bigger risk for mental impairment.

There's also some divergence within the specific categories of harm. Alcohol and heroin both score high for crime. But alcohol's crime risk is due to its tendency to make people more aggressive (and more prone to committing crime), while heroin's crime risk is based on the massive criminal trafficking network behind it.

The analysis doesn't fully account for a drug's legality, accessibility, or how widely a drug is used. If heroin and crack were legal and more accessible, they would very likely rank higher than alcohol. The harm score for marijuana would also likely rise after legalization, but probably not too much since pot use is already widespread.

Since the study only looked at drug use in the UK, some scores would likely vary if Nutt's team conducted a similar analysis in the US. Nutt said meth in particular would likely be scored as more dangerous, since its use is more common in the states.

Still, Nutt is confident that alcohol would be ranked most dangerous in the US. "I don't see how it couldn't be, really," he said.

What other drug experts say about the UK study

The drug policy experts I talked to about Nutt's study generally agreed that his style of analysis and ranking misses some of the nuance behind the harm of certain drugs.

Jon Caulkins, a drug policy expert at Carnegie Mellon University, gave the example of an alien race visiting Earth and asking which land animal is the biggest. If the question is about weight, the African elephant is the biggest land animal. But if it's about height, the giraffe is the biggest. And if the question is about length, the reticulated python is the biggest.

"You can always create some composite, but composites are fraught with problems," Caulkins said. "I think it's more misleading than useful."

The blunt measures of drug harms present similar issues. Alcohol, tobacco, and prescription painkillers are likely deadlier than other drugs because they are legal, so comparing their aggregate effects to illegal drugs is difficult. Some drugs are very harmful to individuals, but they're so rarely used that they may not be a major public health threat. A few drugs are enormously dangerous in the short-term but not the long-term (heroin), or vice versa (tobacco). And looking at deaths or other harms caused by certain drugs doesn't always account for substances, such as prescription medications, that are often mixed with others, making them more deadly or harmful than they would be alone.

Nutt acknowledges these problems, but argues that his analysis provides value to policymakers. "Anyone interested in alcohol and other drugs, from law enforcement to education and from health improvement to international policy, needs some measure that allows them to understand and communicate relative harms and risks," Nutt wrote in his 2011 response to critics. "I believe we have provided the best currently available analysis of an extremely complex multifaceted data set."

So is marijuana really safer than alcohol?

Marijuana is generally safer than alcohol. Drug experts broadly agree that individuals and society would arguably be better off if marijuana became the most accepted recreational intoxicant of choice instead of alcohol.

Health risks are just one way to measure whether marijuana is safer than alcohol. While pot doesn't seem to cause organ failure or fatal overdoses, alcohol kills more than 29,000 people each year due to liver disease and other forms of poisoning.

Alcohol and marijuana are both intoxicants, but one study from Columbia University researchers estimated that alcohol multiplies the chance of a fatal traffic accident by nearly 14 times, while marijuana nearly doubles the risk.

Alcohol's effects on behavior can also lead to more crime, while marijuana use appears to have little-to-no effect. Alcohol is a factor in 40 percent of violent crimes, according to the National Council on Alcoholism and Drug Dependence. But various studies found marijuana doesn't make users more aggressive or lead to crime.

But how much does all of this information really tell policymakers or the public? It would matter if marijuana ends up substituting alcohol once pot is legalized (since a safer substance would be replacing a more dangerous one), but the research on that is still early. And the argument that alcohol is more dangerous than illegal substances could be used as a basis for banning or strictly regulating alcohol just as easily as it could be used as a basis for legalizing or decriminalizing other drugs.

The question policy experts typically ask isn't which drug is more dangerous, but how marijuana and alcohol should be treated through policy as individual drugs with their own set of unique, complicated risks. That doesn't mean just legalization or prohibition, but regulation, taxes, and education as well.

"There's always choices," Keith Humphreys, a drug policy expert at Stanford University, explained. "There is no framework available in which there's not harm somehow. We've got freedom, pleasure, health, crime, and public safety. You can push on one and two of those — maybe even three with different drugs — but you can't get rid of all of them. You have to pay the piper somewhere."

Marijuana isn't perfectly safe

Heavy drug use is never ideal — and marijuana is no different in this regard.

"The main risk of cannabis is losing control of your cannabis intake," Mark Kleiman, a drug policy expert at UCLA, said. "That's going to have consequences in terms of the amount of time you spend not fully functional. When that's hours per day times years, that's bad."

Caulkins of Carnegie Mellon University put it another way: "At some level, we know that spending more than half of your waking hours intoxicated for years and years on end is not increasing the likelihood that you'll win a Pulitzer Prize or discover the cure for cancer."

These problems are compounded by the perception that pot is harmless: Since many marijuana users believe what they're doing won't hurt them, they feel much more comfortable falling into a habit of constantly using the drug.

A lot of research has also linked adolescent marijuana use with a range of negative consequences, including cognitive deficiencies and worse educational outcomes. While it's not clear whether marijuana's role with these outcomes is cause-and-effect, experts generally agree that people younger than their mid-20s should avoid pot.

The research on other health effects of marijuana is inconclusive but should warrant some caution. One study linked the use of potent marijuana to psychotic disorders, but other studies suggest people with psychotic disorders may be predisposed to pot use. Research on whether smoked marijuana causes lung disease or cancer has yielded conflicting results, with studies that control for tobacco smoking finding no significant effect from marijuana on lung cancer risk.

All of this helps prove that marijuana isn't totally harmless — and some of its risks are likely unknown.

How we should evaluate drugs and the harm they do

There probably isn't a perfect way to evaluate and present all drug harms. Researchers will always need to balance making information simple and accessible for policymakers and the public with the inherent complexity of drugs and their effects. This makes the task of building scientific drug policies very challenging.

Some experts say the complexity of the issue should be embraced. Caulkins and Peter Reuter, a drug policy expert at the University of Maryland, suggested a model in which all the major risks of drugs are drawn out and each drug is ranked within those categories. So heroin would be at or near the top for mortality, alcohol would be at or near the top for cause of violent crime, and tobacco would be at the top for long-term health risks. But there wouldn't be a single ranking for all the drugs' harms. The idea is lawmakers could look at this model to help decide on an individual basis which policies are better for each drug.

But it doesn't seem like anyone is taking on this kind of approach — and Nutt's style of analysis remains popular around the world. Although Nutt couldn't get funding to do an analysis in the US or Canada, he said a similar study is being published later this year assessing drug use in several countries in Europe. The analysis may be flawed, but its simplicity and accessibility have won over many policy circles.