Argentina and Mexico have taken significant steps towards decriminalising drugs amid a growing Latin American backlash against the US-sponsored "war on drugs".

Argentina's supreme court has ruled it unconstitutional to punish people for using marijuana for personal consumption, an eagerly awaited judgment that gave the government the green light to push for further liberalisation.

It followed Mexico's decision to stop prosecuting people for possession of relatively small quantities of marijuana, cocaine, heroin and other drugs. Instead, they will be referred to clinics and treated as patients, not criminals.

Brazil and Ecuador are also considering partial decriminalisation as part of a regional swing away from a decades-old policy of crackdowns still favoured by Washington.

"The tide is clearly turning. The 'war on drugs' strategy has failed," Fernando Henrique Cardoso, a former Brazilian president, told the Guardian. Earlier this year, he and two former presidents of Colombia and Mexico published a landmark report calling for a new departure.

"The report of the Latin American Commission on Drugs and Democracy has certainly helped to open up the debate about more humane and efficient policies. But, most of all, the facts are speaking by themselves," said Cardoso.

Reform campaigners have long argued that criminalisation enriched drug cartels, fuelled savage turf wars, corrupted state institutions and filled prisons with addicts who presented no real threat to society.

The US used its considerable influence to keep Latin America and the UN wedded to hardline policies which kept the focus on interdictions and jail sentences for consumers as well as dealers. The "war" was first declared by the Nixon administration.

The economic and social cost, plus European moves towards liberalisation, have emboldened some Latin American states to try new approaches.

Argentina's supreme court, presented with a case about youths arrested with a few joints, ruled last week that such behaviour did not violate the constitution. "Each adult is free to make lifestyle decisions without the intervention of the state," it said.

The government, which favours decriminalisation, is expected to amend laws in light of the ruling. The court stressed, however, that it was not approving complete decriminalisation, a move that would be fiercely resisted by the Catholic church and other groups.

The previous week the government of Mexico, which has endured horrific drug-related violence, made it no longer an offence to possess 0.5g of cocaine (the equivalent of about four lines), 5g of marijuana (about four joints), 50mg of heroin and 40mg of methamphetamine.

Three years ago, Mexico backtracked on similar legislation after the initiative triggered howls of outrage in the US and predictions that Cancún and other resorts would become world centres of narcotics tourism.

Now, however, the authorities quietly say they need to free up resources and jail space for a military-led war on the drug cartels, even while publicly justifying that offensive to the Mexican public with the slogan "to stop the drugs reaching your children". They also argue corrupt police officers will be deterred from extorting money from drug users.

Washington did not protest against the announcement, which was kept deliberately low key. "They made no fanfare so as not to arouse the ire of the US," said Walter McKay, of the Mexico City-based Institute for Security and Democracy. "I predict that when the US sees its nightmare has not come true and that there is no narco-tourist boom it will come under more pressure to legalise or decriminalise."

Some US states have decriminalised the possession of small amounts of marijuana and the Obama administration has emphasised public health solutions to drug abuse, giving Latin America more breathing room, said Kasia Malinowska-Sempruch, director of the Global Drug Policy Programme. "My hope is that Latin America will be the next region, after most of Europe, where evidence and science will be the basis for policy-making."

Argentina and Mexico's moves may encourage other governments to follow suit. A new law has been mooted in Ecuador, where President Rafael Correa last year pardoned 1,500 "mules" who had been sentenced to jail. His late father was a convicted mule.

Brazil's supreme court, as well as elements in Congress and the justice ministry, favour decriminalising possession of small quantities of drugs, said Maria Lúcia Karam, a former judge who has joined the advocacy group Law Enforcement Against Prohibition.

She welcomed the moves towards decriminalisation but said repression remained a cornerstone of drug policy. "Unfortunately the 'war on drugs' mentality is still the dominant policy approach in Latin America. The only way to reduce violence in Mexico, Brazil or anywhere else is to legalise the production, supply and consumption of all drugs."