Flu remedy ContacNT has been used by drug makers in the notorious "drug village" of Boshe in Guangdong Province as a key ingredient to produce methamphetamine, in the most recent revelations from the ongoing investigation.

British pharmaceutical giant GlaxoSmithKline (GSK), who produces the medicine in China, has said it is cooperating with authorities to ensure it is not used illegally, GSK told the Global Times in a statement.

The chemical conversion from the medicine to meth is not complicated, Sun Zhongshi, an expert with the Chinese Monitoring Network for Rational Use of Drugs, told the Global Times.

"It only requires very basic knowledge of chemistry, and can be managed by ordinary people," Sun said. ContacNT's high concentration of pseudoephedrine is the reason lawbreakers prefer it to other flu remedies, he noted.

A ContacNT capsule contains 90 milligrams of pseudoephedrine hydrochloride, a substance that can be manufactured into drugs, and one ContacNT tablet contains 30 milligrams.

In theory, 10 boxes of ContacNT capsules can make 3 grams of meth, China Business News reported on Monday.

People were easily driven by the quick money as a box of ContacNT costs only about 11 yuan ($1.8) and a gram of meth is worth 1,000 yuan.

ContacNT capsule was listed as prescription drug in 2012 and since 2013, it can only be bought in hospitals, Louisa Luo, director of communication with GSK China, told the Global Times Monday.

An earlier report by Nandu Daily said that a primary school student can earn 10,000 yuan per month during the summer and winter holidays by emptying the powder from ContacNT capsules in Boshe.

Empty ContacNT boxes were seen dumped in green areas and the river was dyed black, the report said.

Since 2010, there have been a number of drug rings busted, in which ContacNT was used to make meth, in Shandong, Jiangsu, Hubei and Guangdong provinces.

To deter illegal drug production, China's drug authorities have ordered pharmacies to check and register the identity cards of people purchasing medicines containing ephedrine since September 2012, and each purchase is limited to two to five boxes.