Experts in traditional Chinese medicine are calling for caution over the use of some products that contain cinnabar, or mercury sulfide.

A Sina Weibo user verified as a doctor at Beijing's Jishuitan Hospital wrote that 86 prescriptions of medicine developed by Tongrentang, a leading TCM maker, contain cinnabar.

The mercury content in the chemical can build up in the human body over time and cause damage to organs, including the kidneys, and the nervous system.

The controversy goes back to May 7, when health authorities in the Hong Kong Special Administrative Region recalled a batch of Tongrentang medicine as it contained five times the mercury limit permitted in Hong Kong.

Tongrentang responded by saying its medicines containing cinnabar comply with the national standard, and are safe and effective as long as they are taken according to instructions.

According to the Pharmacopoeia of China, cinnabar can be used to treat insomnia and blurred eyesight. It says that 0.1 to 0.5 grams can be taken at a time, but warns that the ingredient is poisonous and should not be taken in large amounts or over a long time, and pregnant women and people with liver or kidney problems must not use it.

Zhu Qingwen, an assistant professor at Beijing University of Chinese Medicine, also said cinnabar is "a very important material" for making TCM, as it helps people fall asleep. Yet according to Ning's post, sleepiness can be a result of mercury poisoning.

"It needs experiments and data to prove it (whether using cinnabar in TCM causes mercury poisoning)," Zhu said. "All kinds of medicines, including TCM, must go through toxicity tests before getting permission to go on sale."

Cui Yongqiang, chief of the international cooperation department at the China Academy of Chinese Medical Sciences' Guang'anmen Hospital and a senior TCM specialist, said great caution is required with cinnabar.

TCM doctors, who are also usually pharmacists who prepare medicines by blending different processed materials, are becoming more wary about prescriptions containing cinnabar, he said.

"Most TCM doctors only prescribe medicine containing cinnabar made by pharmaceutical companies, instead of directly formulating medicines containing such an ingredient," he said.