Vitamins taken by millions to ward off cancer may accelerate the spread of malignant skin tumours, a study has found.
Scientists urged patients to use the supplements "with caution" after demonstrating the potentially lethal effect of antioxidants.
Tests on laboratory mice with melanoma showed that if they were given an antioxidant supplement in their drinking water it doubled the spread of tumours to the lymph nodes.
In other experiments, human melanoma cells exposed to a chemical mimicking vitamin E - a powerful antioxidant vitamin - became more mobile and invasive.
The research suggests that while antioxidants do not affect tumour growth, they might increase the risk of metastasis - the spread of cancer around the body.
In most cases it is the migration of local cancers to vital organs such as the brain and liver that causes the death of patients.
Antioxidants, which include vitamins E, C and A, are generally seen as powerful healing agents because of their ability to neutralise destructive "free radical" molecules called reactive oxygen species (ROS).
ROS molecules can damage DNA, leading to potentially cancerous genetic changes - but they also form a vital part of the immune's system's arsenal.
Researchers led by Dr Martin Bergo, from the University of Gothenburg in Sweden, issued the new warning in the journal Science Translational Medicine.
They wrote: "Our finding of increased metastasis suggests that cancer patients should use antioxidant supplements with caution."
Previous work by the same team showed that some antioxidants can spur the growth of lung cancer in mice.
Other studies have produced conflicting results, with some indicating an inhibiting effect on cultured cancers.
The mice used by Dr Bergo's team were given an antioxidant called N-acetylcysteine (NAC) in their drinking water.
This had the effect of doubling the number of tumours appearing in the lymph nodes after cancer cells spread from the skin. Cancer spread to the lungs was also increased, to a lesser extent.
However the antioxidant did not affect cancer proliferation.
Exposing seven types of human melanoma cell to the vitamin E analogue Trolox also had no effect on growth but "increased their migration and invasive properties" said the team.
The study showed that both antioxidants activated a protein likely to be involved in promoting metastasis.
The scientists concluded: "These results demonstrate that antioxidants .. play a previously unappreciated role in malignant melanoma progression."
Each year around 13,348 people are diagnosed with malignant melanoma in the UK and 2,148 die from the disease.
The incidence of the most deadly form of skin cancer in Britain is now five times higher than it was in the 1970s.