Iron tablets taken by millions of people could trigger bowel cancer, warns new research.

Two chemicals commonly found in the supplements may fuel the development of tumours, according to a study.

The popular pills have been linked to an increased risk of the disease for more than 20 years.

Now experiments on human cells grown in the lab have found they boost a molecule linked to the deadliest forms.

About six million prescriptions for iron tablets are issued each year in England and Wales alone.

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Many women take the supplement after pregnancy and it is used as a treatment for anaemia, a condition caused by low levels of the mineral.

The international team said although the study was carried out in a lab, rather than living people, it adds to concerns about their safety.

The compounds ferric citrate and ferric EDTA are used in dietary supplements and as a food additive, respectively, across Europe and the US.

Lead author Professor Nathalie Scheers, of Chalmers University of Technology, Gothenburg, Sweden, said: "We can conclude ferric citrate and ferric EDTA might be carcinogenic, as they both increase the formation of amphiregulin, a known cancer marker most often associated with long term cancer with poor prognosis."

The chemicals have previously been shown to worsen tumours in mice with bowel cancer.

But until now the mechanism was little understood, and the possible effects on human cells not investigated.

So Prof Scheers and colleagues, including staff at the UK Medical Research Council and Cambridge University, looked at the effect of normal doses of the compounds found in iron tablets on two types of human colon cancer cells.

As a comparison, they also measured the effects of ferrous sulphate, another very common iron chemical.

While this had no effect both ferric citrate and ferric EDTA caused an increase in cellular levels of amphiregulin, the cancer biomarker - even at low doses.

Today there are many different types of iron supplements on the market based on at least 20 different compounds, and sold under a wide range of brands.

Ferric sulphate is one of the most common, but ferric citrate, which is said to be gentler for the stomach, is also widely available in stores and online.

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It is also more easily absorbed by the body through foods such as granary bread, beans and nuts.

But for consumers looking to make an informed choice, it can often be difficult to know what exactly they are buying.

Prof Scheers said: "Many stores and suppliers don't actually state what kind of iron compound is present - even in pharmacies. Usually it just says 'iron' or 'iron mineral', which is problematic for consumers."

Iron is also added to some foods, to combat deficiency. Ferric EDTA is approved as a fortifying agent in both the US and the EU.

It is also used in countries such as China, Pakistan, Brazil, Mexico and The Philippines, where it is added to flour and powdered drinks.

Additionally, it is present in certain medicines for children with low iron levels in countries such as the UK and France.

Prof Scheers, whose findings are published in Oncotarget, advised against panic but said with both ferric citrate and ferric EDTA in widespread use, there was cause for concern.

She said: "First, we must bear in mind the study was done on human cancer cells cultured in the laboratory, since it would be unethical to do it in humans.

"But, the possible mechanisms and effects observed still call for caution. They must be further investigated.

"At the moment, people should still follow recommended medical advice. As a researcher, I cannot recommend anything - that advice needs to come from the authorities.

"But speaking personally, if I needed an iron supplement, I would try to avoid ferric citrate."

Beyond this, she was not willing to comment as research in the field has so far been limited, even concerning the more common ferrous sulphate.

The key thing for her is that we begin to differentiate between the various forms of iron.

Added Prof Scheers: "Most importantly, researchers and authorities need to start to distinguish between this form of iron and that form of iron. We need to consider that different forms can have different biological effects."

Most of the iron that the body needs is obtained through food such as meat, fish, vegetables, fruits and whole grains. But sometimes this is not enough.

The NHS recommends men need an average of 8.7mg of iron a day, and women who are menstruating need around 14.8mg because of blood loss.

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Pregnant women may also need iron supplements to ensure they get enough for their growing baby.

Two years ago a British study of cells found in blood vessels suggested iron tablets could damage DNA within 10 minutes of swallowing them.

That team said cells may be more sensitive to iron than previously thought.

A 100g of steak has 3 mg of iron, while the same weight of spinach contains 2.7 mg.

But low dose supplements that can be bought over the counter at pharmacies and supermarkets typically contain around 14mg - the equivalent of a day's intake.

Good sources include dark leafy vegetables, brown rice, meat and fish, nuts and seeds eggs and dried fruit.