LONDON - People should stop taking four traditional Chinese medicines on sale in Britain that contain an acid linked to cancer and kidney failure, the drug safety watchdog said on Thursday.

The Medicines and Healthcare products Regulatory Agency (MHRA) said patients who have taken the medicine should see their doctor immediately.

Officials seized 6,500 packets of the product in London and northwest England and have appealed to shops to clear their shelves of any remaining medicine.

It said the four products contain aristolochic acids, a carcinogenic chemical linked to a range of serious illnesses.

"The public is advised not to take any of these products," the MHRA said in a statement. "There is no safe level of exposure to aristolochic acid."

The products are: Xiao Qin Long Wan, a cold and flu medicine; pain reliever Chuan Xiong Cha Tiao Wan; Bai Tou Weng Wan, sold for stomach problems, and Xie Gan Wan, used to treat stress.

The use of the plant species Aristolochicia in unlicensed medicines was banned in the UK in 1999.