To chants of “legal weed boosts morale” and the spectacle of a legislator rolling a joint, Mexico’s lower house of congress passed a sweeping legalisation of marijuana this month. The measure, which President Andrés Manuel López Obrador is expected to sign into law within the next month, will allow Mexicans to grow several marijuana plants at home, purchase cannabis from licensed vendors and possess up to 28 grammes for recreational use.

The consequences are likely to be felt beyond Mexico’s borders. The US, architect of the global war on drugs, finds itself sandwiched between two neighbours that are legalising marijuana (Canada decriminalised it in 2018). More than a dozen US states already allow recreational use of the drug and Senate majority leader Chuck Schumer has pledged to push legislation this year to end the federal prohibition of marijuana.

Advocates of legal marijuana, including several former heads of government, claim a low risk of harm from the drug and cite the benefits of prising the trade from the grip of illegal cartels: the opportunities for regulation, control of quality and taxation, as well as freeing up police for other law enforcement priorities. A previous Mexican president and outspoken supporter of legalisation, Vicente Fox, has put his money where his mouth is and joined the board of a Canadian-Colombian medical cannabis company.

Opponents argue that the health risks of regular marijuana use have been underestimated. They see dangers of legalisation leading to increased addiction and of the drug acting as a gateway to other, more powerful, illegal narcotics. A majority of Mexicans appear to share those views; according to a recent poll, 58 per cent oppose legalisation.

Whatever the pros and cons of Mexico’s marijuana move, experts agree that it is unlikely to dent the enormous power of the drug cartels or their propensity for appalling violence. The country has paid a terrible price for its location as a key shipment point for drugs entering the lucrative US market, with more than 300,000 homicides since 2006, many of them narcotics-related.

Although marijuana was the original illegal crop of choice for Mexican traffickers, they have diversified extensively in recent years into newer, more lethal and more profitable drugs, such as the synthetic opioid fentanyl. Many times more potent than heroin, fentanyl can be produced cheaply and easily using precursor chemicals from Asia and then smuggled across the border. Seizures of fentanyl rose 486 per cent last year in Mexico, and confiscations of other hard drugs such as cocaine and heroin also increased sharply.

Supporters of legalisation argue that addicts to hard drugs should be treated as medical rather than criminal cases — an approach taken by Portugal. Faced with one of Europe’s highest rates of drug-related deaths, the Iberian nation decriminalised possession and use of small amounts of all drugs in 2001, though not their sale or production (deaths have since fallen). But given how dangerous some hard drugs are, even the most ardent decriminalisers struggle to explain how heroin or fentanyl could be produced, sold and used legally and safely. It is also not a given that legalisation eliminates illegal drug-dealing; two years after permitting marijuana sales, Canada has found that underground sellers still thrive because they have a price advantage over (taxed) legal ones.

Legal weed may boost morale for some but the effect on the illegal drug trade — like that on the mind — is likely to be transitory.