Fact check: How dangerous is cannabis, really?

Should cannabis be legal? Several governments — such as in Canada, South Africa and in several US states — have greenlighted recreational use of the substance in recent years. Germany's government has now announced a draft law that would allow limited legalization for adults. Still, in many parts of the world, the possession and consumption of cannabis is prohibited and often severely punished.

Advocates of legal cannabis use in Germany's coalition government aim to legalize the drug this year but will face resistance from conservative lawmakers.

For years, discourse — both online and off — surrounding cannabis legalization has been steeped in falsehoods and claims that, when placed under scrutiny, can't be proven. DW's fact-checking team took a look at the research and spoke with experts to clarify some of the most popular myths.

Is cannabis a gateway drug?

Claim: Cannabis is "a gateway drug" according to, for example, German politician Markus Söder.

Fact check: Unproven.

The theory that using cannabis leads to the use of harder and more dangerous drugs is probably one of the most common cases made against legalization, as well as one of the oldest. Söder and his colleagues used the argument in an interview with the German tabloid newspaper Bildin April 2023, accusing opposition parties of "trivializing" the drug.

On social media, supporters of legalization generally oppose the claim.

Fact: There is a correlation between cannabis use and later use of harder narcotics, studies have shown. The earlier and the more frequently people use cannabis, the higher the likelihood of later use of other illicit drugs.

But correlation is not causation

"If you look at how someone became a heroin user, you are very likely to find cannabis use along the way," said Dr. Stefan Tönnes, head of forensic toxicology at Frankfurt University Hospital. "But if you look the other way around, how many cannabis users go on to use heroin afterwards — it's very, very few." This shows that a correlation alone is not proof of a causal relationship.

The gateway drug theory cannot be completely disproved, said Dr. Eva Hoch, a psychologist in the psychiatry department of the University Hospital in Munich. She has been studying the effects of cannabis for about 20 years.

"Cannabis naturally stimulates the reward center in the brain and could pharmacologically promote drug affinity," she said.

But there are also many other risk factors for illicit drug use that need to be considered, she added. The National Institute on Drug Abuse in the US says so, too, adding that more research is needed on the question of cannabis as a gateway drug.

For the time being, the claim that cannabis is a gateway drug remains unproven.

Is alcohol more dangerous than cannabis?

Claim: Alcohol is "over 100 times more dangerous than cannabis", according to tweets like this one.

Fact check: Misleading.

It is often claimed — especially by those advocating for legalization — that alcohol is far more harmful than cannabis. Several social media posts and newspaper articles mention a study that claims alcohol is 114 times more dangerous than cannabis.

This claim does not hold up — the figure is nowhere to be found in the cited study, which only shows that the risk of a fatal alcohol overdose is greater than that of a fatal cannabis overdose.

Since its effect starts shortly after being smoked, the strength of intoxication caused by cannabis can be better compared to alcohol, Tönnes said, thereby reducing the risk of an overdose. However, "when cannabis is consumed as a pastry, overdoses can occur as well."

With that said, the negative effects of both drugs don't start with overdose. But the study referenced in these posts doesn't address the nonfatal consequences.

The different intoxicating effects of alcohol and cannabis come with their own dangers, Tönnes said, adding that it's important to take into account the effects on the user's social environment and mental health.

"Alcohol has a very significant effect, which is disinhibition and increased risk-taking," he said. "This is actually less the case with cannabis. But here we have the unpredictable paranoia risk, and that's where individual sensitivity to cannabis effects can vary."

The negative effects of alcohol consumption on the body have long been proved. "Alcohol has a high organ-damaging effect and causes more health damage than cannabis," Hoch said. But "it also depends on the intensity of use, not just the substance."

It's hard to clearly assess the health risks of cannabis because of its various forms of use, Hoch said. In Europe, for example, cannabis is often smoked with tobacco, which is well-known to have harmful carcinogenic effects — this is thus indirectly related to cannabis use.

So alcohol overdoses are more likely than cannabis. But both drugs can endanger the physical and mental health of users and have a negative impact on their environment, even in small quantities. This danger can hardly be compared with one number — the claim that alcohol is 114 times more dangerous than cannabis is therefore misleading.

Can you die from excessive cannabis use?

Claim: According to this tweet, there has been "no recorded death due to cannabis overdose."

Fact check: True.

According to NIDA, no single overdose death that has occurred so far has been attributed solely to cannabis. The US Centers for Disease Control and Prevention also states that "a fatal overdose is unlikely."

Despite this, cannabis use as a cause of death remains under discussion and the subject of scientific studies.

In the 1970s, tests were carried out on dogs and monkeys to determine what dose of cannabis could be potentially lethal. The animals were administered a high oral dose of tetrahydrocannabinol (THC). They showed symptoms such as drowsiness, tremors and vomiting, but all survived the involuntary high.

Animal studies are difficult to transfer to humans, but Hoch said "the lethal dose for cannabis is very, very high. It's very unlikely that a human would ingest that much."

Cannabis: Death not ruled out

Researchers at King's College London have been trying to retrospectively determine whether cannabis can be deadly to humans by reviewing all cannabis-related deaths that occurred in England between 1998 and 2020.

In almost all cases, cannabis was not the only drug — on average, three to seven other substances were also detected, such as opiates, alcohol or drugs such as tranquilizers or sleeping pills.

In 4% of the deaths, researchers found cannabis to be the sole cause of death, usually due to injuries sustained during intoxication. In one case, cannabis toxicity could have resulted in death. Still, it remains unclear whether a single dose was the cause of death or the duration of use ultimately contributed.

Hoch said there were "other published cases of cannabis-related deaths." These have been reported in association with accidents, suicides or cardiovascular complications such as heart attack. But establishing the cause of these deaths is difficult, she said.

Smoking pot is not good for your heart

The NIDA also warns of the risks that can result from the elevated heart rate caused by cannabis use. The institute has stated that "marijuana increases heart rate for up to three hours after smoking," which it says may increase the risk of heart attack.

"Cannabis definitely has an effect on the cardiovascular system," Tönnes said. "People who are particularly sensitive, have a predisposition or have previous damage may therefore be particularly sensitive to cannabis."

Older studies also suggest a link between cannabis use and an increased risk of cardiovascular disease. "You can't rule out the possibility that this could become life-threatening," Tönnes said.

Why is a single overdose (likely) not fatal?

The risk of dying from a single cannabis overdose is "negligible," the King’s College researchers concluded.

Apart from the huge amount of cannabis needed, it is also considered physiologically unlikely, Hoch said. The very low presence of cannabinoid receptors in the brain stem explains why cannabis — at least in people without preexisting conditions — has a less potent effect on breathing or other important bodily functions such as blood pressure or heart rate. Opioid receptors, on the other hand, play a greater role in the brain stem, which is why a heroin overdose can lead to respiratory failure.

"Alcohol can also have a paralyzing effect on the central nervous system, especially the respiratory center," Tönnes said, "and therefore lead to death."

Does cannabis kill brain cells?

Claim: "Marijuana doesn't kill brain cells," according to some Twitter accounts.

Fact check: Unproven.

How does cannabis or marijuana (as the dried, resinous flowers and the small leaves of the female hemp plant near the flower are called) affect the brain? In the 1970s, the psychiatrist Robert Heath, of Tulane University Medical School in New Orleans, made headlines after conducting an experiment on rhesus monkeys that he said proved that marijuana use kills brain cells.

The qualitative implementation of the experiment was heavily criticized, and the results of the study were later refuted by researchers at the National Center for Toxicological Research in Arkansas. That was just the beginning.

Many hypotheses, no clarity

To date, studies on the long-term effects of cannabis on brain structure in humans have shown conflicting results.

Hoch has long followed the "explosion of publications" around cannabis.

"It's true that cannabis interferes with neurophysiology," she said. But she emphasized that even she can't clearly say how neurotoxic or brain-damaging the main active ingredient in cannabis, THC, really is. She said further research was needed.

More is known about the immediate, shorter-term effects. There is good evidence that certain mental abilities are impaired after acute cannabis use, such as short-term memory, psychomotor coordination or attention span. With chronic use, these effects can persist for days. However, they appear to be reversible after several weeks of abstinence.

Young people at risk

There is no denying that cannabis use can damage young brains in particular, as the brain develops significantly during the transition from childhood to adulthood, Hoch said.

The US Centers for Disease Control and Prevention warns that marijuana use before age 18 can affect how the brain builds connections for functions such as attention, memory and learning, but also emphasizes that the effects of marijuana use on the brain depend on many factors, such as the amount of THC, how often it is used, age at first use and whether use is accompanied by other substances, for example, alcohol or tobacco.

Long-term effects on the brain may also be caused by factors other than marijuana, such as genetics, home environment or other unknown factors.

Studies needed on neurogenesis through cannabis

A 2019 study caused a stir when it found that after smoking marijuana, teenagers have more gray matter in certain areas of the brain. But not even the researchers involved wanted to take a clear position on whether the increase in gray matter should be seen as beneficial or rather harmful. The data has to be interpreted with caution.

Hoch said the claim that cannabis can grow new brain cells was interesting and should be pursued in the future.

Initial animal experiments have focused on the effect of cannabidiol (CBD) on neurogenesis. CBD, along with THC, is one of the best-known cannabinoids, or chemical compounds found in cannabis. But there's a difference: THC has a psychoactive effect and CBD does not.

Further research into the body's own cannabis system is needed, Hoch said. "When do cannabinoids promote health? When do they pose risks?," she asked. So far, more than 140 cannabinoids have been discovered. For most of them, the effects have not even been studied, she said.

"Cannabis products available on the black market have a completely unclear cannabinoid profile. They usually contain very little CBD, but a lot of the main intoxicating active ingredient THC. Dangerous additives such as synthetic opioids or cannabinoids could also be included," Hoch said. That's why her advice to teens is: "You're doing your brain a favor by not smoking pot".