"All right!" said a grinning Ivan Rojas, a rail-thin 20-year-old addict who endured police harassment during the decade he has spent sleeping in Mexico City's gritty streets and subway stations.
But stunned police on the U.S. side of the border say the law contradicts President Felipe Calderon's drug war, and some fear it could make Mexico a destination for drug-fueled spring breaks and tourism.
Tens of thousands of American college students flock to Cancun and Acapulco each year to party at beachside discos offering wet T-shirt contests and all-you-can-drink deals.
"Now they will go because they can get drugs," said San Diego Police Chief William Lansdowne. "For a country that has experienced thousands of deaths from warring drug cartels for many years, it defies logic why they would pass a law that will clearly encourage drug use."
Enacted last week, the Mexican law is part of a growing trend across Latin America to treat drug use as a public health problem and make room in overcrowded prisons for violent traffickers rather than small-time users.
Brazil and Uruguay have already eliminated jail time for people carrying small amounts of drugs for personal use, although possession is still considered a crime in Brazil. Argentina's Supreme Court ruled out prison for pot possession on Tuesday and officials say they plan to propose a law keeping drug consumers out of the justice system.
Colombia has decriminalized marijuana and cocaine for personal use, but kept penalties for other drugs.
Officials in those countries say they are not legalizing drugs — just drawing a line between users, dealers and traffickers amid a fierce drug war. Mexico's law toughens penalties for selling drugs even as it relaxes the law against using them.
"Latin America is disappointed with the results of the current drug policies and is exploring alternatives," said Ricardo Soberon, director of the Drug Research and Human Rights Center in Lima, Peru.
As Mexico ratcheted up its fight against cartels, drug use jumped more than 50 percent between 2002 and 2008, according to the government, and today prisons are filled with addicts, many under the age of 25.