(Corrects seventh paragraph to show U.S. does not require prescriptions)

By Noel Randewich

MEXICO CITY, July 19 (Reuters) - Everyday cold medicines in Mexico will be moved behind the counter from September to stop drug gangs from using them to make illegal narcotics.

Many over-the-counter decongestants in Mexico contain pseudoephedrine, a chemical that can be used to make highly addictive methamphetamine, often known as crystal meth.

"Establishments that openly sell these kinds of classified medicines can continue until August 31," the health ministry said in a statement late on Wednesday.

On Thursday, some Mexico City pharmacies were already refusing to sell cold medicine without a prescription.

Methamphetamine production is booming in Mexico. Superlabs have sprung up to supply the U.S. market after a crackdown shut many U.S. meth labs, according to the U.S. government.

Labs south of the border have also benefited from easier access in Mexico to precursor chemicals like ephedrine and pseudoephedrine.

In the United States, prescriptions are not required but buyers must ask pharmacist for medicines containing pseudoephedrine.

Agreements with pharmaceutical companies to stop using pseudoephedrine mean Mexico will import only 33 tonnes of the substance this year, down from a planned 40 tonnes, the health ministry said.

Despite the new rules, many pharmacies in Mexico routinely sell prescription medicines over the counter.

Imports of pseudoephedrine and similar chemicals are already meant to be tightly controlled in Mexico, but several tonnes per year are thought to be imported undetected.

Last December, police seized 20 tons of ephedrine in the Pacific coast port of Lazaro Cardenas.

In March, police found $206 million -- Mexico's biggest cash haul, in a raid on a Mexico City house belonging to a man accused by the government of being a meth manufacturer.

That raid was the most high-profile bust of President Felipe Calderon's nationwide crackdown on drug cartels.

Usually smoked or injected, crystal meth can quickly become addictive, leading to depression, paranoia, violence, kidney failure and internal bleeding.

Most medicine manufacturers in Mexico have agreed to replace pseudoephedrine in cold remedies with equally effective drugs, the health ministry said.

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