Taking vitamin supplements could increase the risk of developing cancer and heart disease by a fifth, new research has warned.
The decade-long study examined the healthy benefits of supplements on thousands of people and found they can do "more harm than good."
The revelations will raise questions over the market for shop-bought vitamins, which according to market research group Mintel was estimated to be worth £385 million in 2012.
Experts said exceeding the recommended daily amount of over-the-counter vitamins may increase the risk of developing cancer and heart disease by up to 20%.
They urged the public to get their vitamins from a healthy diet rather than rely on pills.
Research by the Food Standards Agency in 2008 estimated one in three Britons took some form of dietary supplement and half of all households with children gave them vitamins or minerals.
The research team began investigating the potential health benefits of vitamins and minerals 20 years after observing that people who ate more fruits and vegetables tended to have less cancer. Scientists wanted to see if taking supplements would produce the same effects.
But they were surprised to discover that far from boosting people's health, taking too much of a vitamin supplement can increase the risk of potentially fatal diseases.
Dr Byers said the findings did not suggest that all vitamin supplements were dangerous and should be shunned. But he warned that taking too many of them can carry a health risk.
One trial exploring the effects of beta-keratin supplements showed that taking more than the recommended dosage increased the risk for developing both lung cancer and heart disease by 20%.
Folic acid, which was thought to help reduce the number of potentially cancerous growths known as polyps in a colon, actually increased the number in another trial.