More than five joints a week entail a high health risk, according to researchers

Despite cannabis being considered a soft drug, explain the scientists, more than this amount can be detrimental

A man lights a marijuana cigarette
A young man smokes a joint in New York.Eduardo Muñoz Álvarez (VIEWpress / Corbis / Getty)
Jessica Mouzo

Cannabis is usually classified as one of the so-called soft drugs, those that are socially accepted and perceived as less harmful, like tobacco or alcohol. However, there are no harmless drugs. All have an impact on physical and mental health and entail a high risk of dependence. However, scientists have tried to pinpoint the threshold that triggers the actual risk; in the case of cannabis, researchers from the Hospital Clínic and the August Pi i Sunyer Biomedical Research Institute (IDIBAPS), in Barcelona, Spain, have reached a scientific consensus according to which five or more joints of marijuana or hashish per week are considered harmful.

According to the World Health Organization (WHO), approximately 147 million people (2.5% of the world’s population) consume cannabis. It is, by far, the most cultivated, trafficked and used illicit substance; cocaine or opiates, in comparison, are consumed by around 0.2% of the world’s population. And even though it is called a soft drug, and despite its therapeutic effects — to treat nausea and vomiting in advanced stages of cancer or AIDS, for example — its harmful effects are numerous: it impairs cognitive development, memory, psychomotor functions and the attention span, and it can cause respiratory lesions, among other damages.

Mercé Balcells, head of the addictive behaviors unit at Hospital Clínic and member of the group that reached this the consensus, pointed out that cannabis use is by no means harmless. “There is no zero risk. You could smoke a joint for the first time today and have a panic attack, for example. You can be a healthy person, smoke a joint and have things happen to you. I can’t tell you that nothing will happen to you. It is a substance that reaches your brain,” she said.

A man rolls a joint.
A man rolls a joint.Leonardo Álvarez Hernández (Getty Images)

Beyond each particular case, in the field of public health a pattern of risk consumption is usually established, the tipping point after which the risk of having a problem related to the consumption of a substance grows exponentially. With alcohol, for example, it is 20 grams a day for men (two glasses of wine) and 10 grams for women. With cannabis, according to the researchers, which presented the results at a scientific conference in Granada, it is five or more joints a week.

Balcells pointed out that quantity is taken into account when measuring risk, but also frequency. She explained that there has been a certain social trivialization of the risks of cannabis, and warns of its complexity: “There is a tendency to present it as something benign, natural... But the fact that it comes from a plant doesn’t mean that it will not have health repercussions.”

The strength of the cannabis is also a key element in calculating risk, added the specialist. “In 2014 we defined a standard joint unit and looked at the amount of THC, which is what causes the damage. The potency of marijuana is important: if it is more than 10%, it is risky consumption,” she said. However, she admitted that it is “difficult” for users to know how much THC is in their joint. “What we see now is that the potential of what is being distributed is increasing. Before, a few years ago, it had a lesser amount,” she warned. More THC means more addictiveness and a greater risk of mental health disorders such as psychosis, or an earlier onset of other ailments like schizophrenia.

Very vulnerable groups

Balcells also highlighted that there are especially vulnerable populations, such as people under 21, pregnant women, nursing mothers or people with underlying physical or mental conditions. For these groups, any consumption, no matter how limited or occasional it may be, poses a health risk. “Consumption at an early age lowers the IQ and produces cognitive alterations.”

The new consensus is in line with the one reached by Canadian researchers, who first of all recommend total abstinence to reduce the risks. These experts pointed out that an early start — consuming before the age of 16 — is associated with “multiple subsequent adverse health and social effects in young adult life,” especially if the consumption is also frequent. They also considered “individuals with predisposition for, or a first-degree family history of, psychosis and substance use disorders, as well as pregnant women (primarily to avoid adverse effects on the fetus or newborn)” to be especially vulnerable to the risks of cannabis.

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