New study suggests that exceeding the recommended daily amount of vitamin supplements can increase the risk of heart disease and cancer.
Dr Tim Byers, of the University of Colorado Cancer Centre, said: 'We are not sure why this is happening at the molecular level.
'But evidence shows that people who take more dietary supplements than needed tend to have a higher risk of developing cancer.'
Two decades ago, research found people who ate more fruits and vegetables tended to have less cancer.
So Dr Bryers began to investigate if taking extra vitamins and minerals would further reduce the risk of developing the disease.
Dr Bryers said: 'This is not to say that people need to be afraid of taking vitamins and minerals.
'If taken at the correct dosage, multivitamins can be good for you.
'But there is no substitute for good, nutritional food.'
He added that most people get the daily recommended doses of vitamins and minerals in their diets by eating healthy meals.
He concluded: 'At the end of the day we have discovered that taking extra vitamins and minerals do more harm than good.'
The findings were presented at the American Association for Cancer Research Annual Meeting 2015.