The only step remaining was the signature of President Vicente Fox, whose office indicated he would sign the bill, which Mexican officials hope will allow police to focus on large-scale trafficking operations rather than minor drug busts.
"This law gives police and prosecutors better legal tools to combat drug crimes that do so much damage to our youth and children," said Fox's spokesman, Ruben Aguilar.
If Fox signs the measure and it becomes law, it could strain the two countries' cooperation in anti-drug efforts — and increase the vast numbers of vacationing students who visit Mexico.
Oscar Aguilar, a Mexico City political analyst, said Fox appeared almost certain to sign the law — his office proposed it, and his party supports it — and that he had apparently been betting that it would not draw much notice.
"That's probably why they (the Senate) passed it the way they did, in the closing hours of the final session," Aguilar said. "He's going to sign it ... he's not going to abandon his party two months before the (presidential) election."
U.S. officials scrambled to come up with a response to the bill.
"The United States and Mexico have a strong history of counternarcotics cooperation, and the Fox administration has taken a firm stand against illegal drug cultivation, trafficking and abuse," said Janelle Hironimus, a State Department spokeswoman. She said the department was trying to get "more information" about the measure.
One U.S. diplomat who requested anonymity because he was not authorized to speak publicly said "any effort to decriminalize illegal drugs would not be helpful."
The bill, passed in the early morning hours by Mexico's Senate on a 53-26 vote with one abstention, has already been approved in the lower house of Congress. It also stiffens penalties for trafficking and possession of drugs — even small quantities — by government employees or near schools, and maintains criminal penalties for drug sales.
The bill says criminal charges will no longer be brought for possession of up to 25 milligrams of heroin, 5 grams of marijuana (about one-fifth of an ounce, or about four joints), or 0.5 grams of cocaine — the equivalent of about 4 "lines," or half the standard street-sale quantity (though half-size packages are becoming more common).
"No charges will be brought against ... addicts or consumers who are found in possession of any narcotic for personal use," according to the Senate bill, which also lays out allowable quantities for an array of other drugs, including LSD, ecstasy and amphetamines.
Some of the amounts are eye-popping: Mexicans would be allowed to possess more than two pounds of peyote, the button-sized hallucinogenic cactus used in some native Indian religious ceremonies.
Mexican law now leaves open the possibility of dropping charges against people caught with drugs if they are considered addicts and if "the amount is the quantity necessary for personal use." The new bill drops the "addict" requirement — automatically allowing any "consumers" to have drugs — and sets out specific allowable quantities.
Mexican officials declined to explain how the law would work — including whether drug use in public would be tolerated, or discouraged by other means.
The law was defended by Mexican legislators — and greeted with glee by U.S. legalization advocates.
"We can't close our eyes to this reality," said Sen. Jorge Zermeno, of Fox's conservative National Action Party. "We cannot continue to fill our jails with people who have addictions."
Ethan Nadelmann, director of the New York-based Drug Policy Alliance, said the bill removed "a huge opportunity for low-level police corruption." In Mexico, police often release people detained for minor drug possession, in exchange for bribes.
Selling all these drugs would remain illegal under the proposed law, unlike the Netherlands, where the sale of marijuana for medical use is legal and it can be bought with a prescription in pharmacies. While Dutch authorities look the other way regarding the open sale of cannabis in designated coffee shops — something Mexican police seem unlikely to do — the Dutch have zero tolerance for heroin and cocaine. In both countries, commercial growing of marijuana is outlawed.
In Colombia, a 1994 court ruling decriminalized personal possession of small amounts of cocaine, heroin and other drugs.
The effects in Mexico could be significant, given that the country is rapidly becoming a drug-consuming nation as well as a shipment point for traffickers, and given the number of U.S. students who flock to border cities or resorts like Cancun and Acapulco on vacation.
"This is going to increase addictions in Mexico," said Ulisis Bon, a drug treatment expert in Tijuana, where heroin use is rampant. "A lot of Americans already come here to buy medications they can't get up there ... Just imagine, with heroin."
Drug policies of selected countries based on information from the New York-based Drug Policy Alliance and governments:
By MARK STEVENSON