The case of a man who developed cancer after taking a Chinese liver tonic highlights the dangers of unregulated herbal therapies, British doctors say.

A 30-year-old Chinese man was referred to a kidney clinic at London's Whittington Hospital in 2003 after showing blood in his urine.

The man had been taking the Chinese herb Longdan Xieganwan to "enhance" his liver, Dr. Robin Woolfson and his co-authors wrote in Saturday's issue of The Lancet.

An active ingredient in Longdan Xieganwan is aristolochic acid, which is recognized as a potent carcinogen.

The man developed a bladder tumour that was surgically removed.

He stopped taking the herb, but when doctors last saw him in June 2006, he had developed recurrent bladder tumours and kidney failure requiring dialysis.

Tonic still available

"Aristolochic acid has been banned in many countries but continues to be available on the internet," the researchers said.

"This case emphasizes the importance of an adequate environmental and dietary history in [urinary/kidney] disease and highlights the dangers of unregulated herbal therapy."

Since 1993, when more than 100 people at a Belgian diet clinic suffered kidney damage after being exposed to aristolochic acid, regulators in the United States, Canada, Britain, Malaysia and other countries have banned aristolochic acid or issued strict warnings about it.

In 2005, Health Canada repeated its warning not to use products containing aristolochic acid.

Aristolochic acid is not authorized for sale in Canada, but travellers returning to the country can bring it in for personal use, the advisory said.

Longdan Xieganwan is manufactured by China Tong Ren Tang, China's oldest supplier of traditional medicine.

Recently, more than 100 Chinese kidney patients started a class-action lawsuit against the company in the country's first consumer action, the British team said, citing a Chinese newspaper report.

A journal editorial accompanying the case report said complementary medicines, like any medicines, have the potential for side-effects, drug interactions and contamination and need to be regulated.