Farmer said the Mexican meth is often made without pseudoephedrine, an ingredient commonly found in cold and allergy pills, which has been banned in Mexico and restricted in the United States. Most meth made in clandestine U.S. labs is made with pseudoephedrine, making it a more powerful high, he said.
"Meth users prefer domestic dope," Farmer said. "What they end up using is a combination of both. They'll use the local dope for special occasions, but when it comes to feeding their habit, they'll revert back to Mexican meth."
The Mexican army said troops received several anonymous tips and found the big drug stash in the township of Tlajomulco de Zuniga, near the Jalisco state capital of Guadalajara. The army statement said that "the historic seizure (is) the most important in terms of quantity of methamphetamines (seized) at one time."
The previous biggest bust announced by the army came in June 2010, when soldiers found 3.1 metric tons (3.4 tons) of pure meth in three interconnected warehouses in the central state of Queretaro, along with hundreds of tons of precursor chemicals used to make meth. A giant underground lab was also found in Sinaloa state.
Those other seizures were believed to be linked to the Sinaloa cartel.
The size of the Jalisco bust stunned Steve Preisler, an industrial chemist who wrote the book "Secrets of Methamphetamine Manufacture" and is sometimes called the father of modern meth-making.
"I have never seen quantity in that range," Preisler wrote. But he added: "The amounts of precursors they were importing would produce multi-tons of product."
Preisler was referring to the dramatic increase in seizures in Mexico of chemicals used to make methamphetamine, usually imported from countries such as China.
In December alone, Mexican authorities seized 675 tons of a key precursor chemical, methylamine, that can yield its weight in uncut meth. All of the shipments were headed for Guatemala, where the Sinaloa cartel is also active. Officials in Guatemala, meanwhile, seized 7,847 barrels of precursors in 2011, equivalent to about 1,600 tons.
The supply of methamphetamine in the United States has been growing, mainly due to its manufacture in Mexico, according to U.S. drug intelligence sources.
Between 2007 and 2009, seizures of methamphetamine by U.S. authorities along the Mexican border increased by 87 percent, according to the 2011 U.N. World Drug Report, the most recent statistics the U.N. has available.
Eighty percent of the meth caught being smuggled into the U.S. is seized at the Mexican border, according to the National Drug Intelligence Center.
Few drugs do as much widespread damage — both to users and the general public — as meth, which is highly addictive. It's produced with volatile chemicals that can lead to explosions.
Chronic use can lead to psychosis, which includes hearing voices and experiencing hallucinations. The stimulant effect of meth is up to 50 times longer than cocaine, experts say, so users stay awake for days on end, impairing cognitive function and contributing to extreme paranoia.
Users are known to lose massive amounts of weight, suffer scabs on their bodies and even lose teeth to "meth mouth" caused when saliva dries up and decay takes over.
Associated Press writers E. Eduardo Castillo in Mexico City, Tracie L. Cone in Fresno, California, and Jim Salter in St. Louis, Missouri, contributed to this report.
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