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By Noel Randewich

MEXICO CITY, July 19 (Reuters) - Some everyday cold medicines in Mexico will be modified or require a prescription starting in September to stop drug gangs from using them to make illegal drugs.

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.

Pharmacies have until Aug. 31 to use up stocks of cold remedies that contain pseudoephedrine, and many drug companies are removing the ingredient from their products altogether, senior health ministry official Gustavo Olais said.

"Only a handful of pharmacies in the country are going to be able to sell medicines that contain pseudoephedrine," he told Reuters on Thursday.

He said international pharmaceutical firms like Pfizer Inc. <PFE.N> that operate in Mexico are taking pseudoephedrine out of their cold remedies.

About 275 medicines are being modified or will only be available with a prescription, Olais said.

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.

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 pharmacists 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.

Patients with a prescription will only be able to buy medicine containing pseudoephedrine in stores that meet strenuous record-keeping requirements.

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

In December, police seized 20 tonnes 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.

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