Scientists in China have demonstrated how arsenic - a favourite murder weapon in the Middle Ages - destroys deadly blood cancer by targeting and killing specific proteins that keep the cancer alive.

"Our study showed how arsenic directly targets these proteins and kills them," lead researcher Zhang Xiaowei at the State Key Laboratory of Medical Genomics in Shanghai, China, said.

"Unlike chemotherapy, the side effects of arsenic (in treating acute promyelocytic leukemia) are very low. There is no hair loss or suppression of bone marrow (function). We are interested in finding out how arsenic can be used in other cancers," Mr Zhang said.

Well known for its toxicity, arsenic was regarded in the past as the king among poisons because its symptoms are like those of cholera and can often go undetected.

In China, however, it has long served a dual purpose. Apart from intentional poisoning, it has been used for at least 2,000 years in traditional Chinese medicine.

In 1992, a group of Chinese doctors reported how they used arsenic to treat acute promyelocytic leukemia (APL), a blood and bone marrow cancer that has surprisingly high cure rates of over 90 per cent in China.

However, the actual workings of arsenic and how it interacts with cancer tissues has never been clear - until Zhang and his colleagues used modern technology to find out.

In a paper published in the journal Science, Mr Zhang and his team, which includes Health Minister Chen Zhu, described how they used modern equipment and saw how arsenic attacked specific proteins that would otherwise be keeping the cancer alive and well.

"This shows how Western technology can be used to find out about the mysteries of Chinese medicine," Mr Zhang said.

"Although many countries are now using arsenic to treat APL, some countries are resistant to the idea. It depends a lot on whether doctors recommend it and whether patients accept it."

In APL, there is a drop in the production of normal red blood cells and platelets, resulting in anemia and thrombocytopenia. The bone marrow is unable to produce healthy red blood cells. Until the 1970s, APL was 100 per cent fatal and there was no effective treatment.