Ruling in a case involving several young men caught with marijuana cigarettes in their pockets, the judges struck down a law providing for up to two years in prison for possession of small amounts of narcotics.
Tuesday's decision doesn't legalize drug possession outright. But Argentina's Cabinet chief favors keeping drug addicts out of the justice system, and was waiting for the ruling before forwarding a proposed law to Congress.
The seven judges said they were unanimous in "declaring the unconstitutionality of prison for private consumption."
"Each individual adult is responsible for making decisions freely about their desired lifestyle without state interference," their ruling said. "Private conduct is allowed unless it constitutes a real danger or causes damage to property or the rights of others."
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Cabinet chief Anibal Fernandez declared that the ruling brings an end to "the repressive politics invented by the Nixon administration" in the United States, and later adopted by Argentina's dictators, to imprison drug users as if they were major traffickers.
On the other hand, Argentina will insist that "those who sell trash to poison our children must be punished with all the power of the state," Fernandez said in an interview with radio Continental.
The ruling sets a precedent that goes beyond marijuana by striking down an article in Argentina's drug law that applies to people caught with personal use amounts of any narcotic.
The judges urged Argentina's government to "create government policies against illegal drug trafficking and adopt preventive health measures, with information and education against drug consumption directed at the most vulnerable groups."
Fernandez announced this month that the proposed law would be ready by year's end. Details have not been made public, and it remains unclear, for example, whether addicts will be forced to get treatment or go to jail.
Opponents say Argentina isn't ready to require treatment for all addicts, since the existing government centers have long waiting lists.