The substance was found to induce remission in patients who had relapsed with a severe form of leukaemia, according to research published in the New England Journal of Medicine.

Two years ago Chinese researchers reported that low doses of a compound of the poison - arsenic trioxide - could aid the recovery of patients with acute promyelocytic leukaemia (APL).

APL is a potentially fatal type of cancer that affects the blood and bone marrow.

Now those results have been born out by the first western pilot study, carried out by US scientists.

Alternative therapy

Researchers at the Memorial Sloan-Kettering Cancer Center in New York treated 12 patients with arsenic.

The patients had all failed to respond to conventional therapy, but with the new treatment 11 achieved remission lasting between 12 and 39 days and suffered only mild side-effects.

The poison is more at home in an Agatha Christie novel

The exception was a patient who died from a complication related to the disease five days after the arsenic treatment began.

Once remission was achieved, each patient had a brief break followed by repeated courses of arsenic every three to six weeks.

After two cycles of therapy highly sensitive tests were performed on the patients to see if there were any molecular signs of leukaemia left.

Three patients tested positive and later relapsed, while eight patients tested negative and remained in remissions that lasted as long as 10 months.

Several patients have so far received up to six courses of arsenic treatment without experiencing cumulative side-effects.

Dr Raymond Warrell, one of the scientists involved in the study, said: "We now know that arsenic can safely bring patients with APL into remission, which may ultimately give them a second chance of life."

The researchers said their findings proved that arsenic kills the cancerous cells that cause APL.

This included those cells that have become resistant to the most successful conventional form of treatment, a drug called all-trans retinoic acid.

Drug resistance

Dr Warrell had previously shown that all-trans retinoic acid used in combination with chemotherapy could be used to good effect as it forced cancer cells to mature and die naturally.

But 30% of those who receive the treatment develop resistance to it and relapse with the disease.

Lead author Dr Steven Soignet said arsenic appeared to exceed the effectiveness of any single drug to treat the disease, and could be used to treat it from the moment of diagnosis.

"Still," he added, "this is not a cure. More studies will tell us how truly effective arsenic trioxide will be over the long term."

Clinical trials using arsenic are on-going at Memorial Sloan-Kettering and several other centres throughout the US.

The investigators now plan to look at the poison's effectiveness in treating other types of cancers.