Lowering cholesterol with statins may slightly increase the risk of cancer, a study suggests.

It is not clear whether the cancer cases are caused by the drugs, or are a consequence of the low levels of "bad" LDL cholesterol produced by taking them.

The result, which amounts to one extra case of cancer for every 1,000 patients treated, surprised the researchers who discovered it. They were looking for new evidence on the known side-effects of statins on the liver and muscle wasting.

"This analysis doesn't implicate the statin in increasing the risk of cancer," said the study leader, Professor Richard Karas, of Tufts University School of Medicine, in Boston. "The demonstrated benefits of statins in lowering the risk of heart disease remain clear. However, certain aspects of lowering LDL with statins remain controversial and merit further research."

The team reviewed the results of 13 previous trials, involving more than 41,000 patients and all published before November 2005. They detected higher rates of cancer among the patients whose use of statins achieved the lowest levels of LDL cholesterol.

This may be important because recent statin trials have shown that a more aggressive lowering of LDL produces greater benefits to the heart. There are moves to lower the cholesterol targets aimed at by GPs, on the assumption that doing so will do no harm. But there have been suggestions that there may be a greater risk of side-effects if a more aggressive statin treatment is used.

The researchers, who published their findings in the Journal of the American College of Cardiology, found that the degree of damage to the liver increased with greater statin doses, but that there was no such effect in muscle wastage. They said the best strategy may be to combine statins at moderate doses with other drugs.

As for cancer, conclusions are difficult to draw. No single form of cancer predominated, so if there is a side-effect of having a very low level of LDL, it would have to apply to all types of cancer. And previous statin trials have not shown any direct effect on cancer risk. But those trials did not compare cancer risk with the degree of lowering of LDL cholesterol.

John LaRosa, of the State University of New York, cast doubt on the findings. If they were caused by a lowering of cholesterol, the effect must have been very rapid, as the trials lasted five years or less. Other explanations, he said, were chance, or simply that people who would otherwise have died of heart disease were living longer, and dying of cancer.

June Davison, cardiac nurse for the British Heart Foundation, said: "We have known about the association between low cholesterol levels and cancer for some time now. While this [research] highlights an association between low levels of LDL and cancer, this is not the same as saying that low LDL or statin use increases the risk of cancer. There is overwhelming evidence that lowering LDL cholesterol through statins saves lives by preventing heart attacks and strokes. These findings do not change the message that the benefits of taking statins greatly outweigh any potential risks. People should not stop taking statin treatment on the basis of this research."