Iron tablets taken by millions of people could damage the body within just 10 minutes, a study has warned.
Tests showed the mineral rapidly causes DNA damage in blood vessels.
While they were carried out in a lab setting, rather than living people, researchers found the levels of iron given in supplements may be too high and harmful.
Iron supplements may contain 10 times more than is necessary for health, the researchers said.
In future, doctors may want to think carefully about prescribing the minimum dose of iron necessary to patients who need it, researchers said.
Iron is an essential element for life. 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.
There were six million prescriptions issued each year for iron tablets in England and Wales alone.
Dr Claire Shovlin, senior author of the study, at the National Heart and Lung Institute at Imperial College London told MailOnline that men need an average of 8.7mg of iron a day, and women who are menstruating need around 14.8mg a day.
A 100g of steak has 3 mg of iron, while the same weight of spinach contains 2.7 mg.
It would be extremely difficult for a person to consume more than 20mg of iron from their diet, Dr Shovlin said.
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.