just one glass of grapefruit juice is enough to treble the potency of some cancer, heart, cholesterol and painkilling drugs.
More than 20 years ago doctors discovered that grapefruit makes some prescriptions dangerously strong. But since then, the list of medicines affected by the fruit has soared.
Scientists have only recently worked out how grapefruit makes drugs more potent.
The problem lies in chemicals in the fruit called furanocoumarins. These are produced by plants often as a defence mechanism against predators.
The concentrations in grapefruit are not toxic, but they interfere with a chemical naturally produced in our intestines called cytochrome P450 3A4.
This enzyme sits in the lining of the intestines and is involved in the metabolism of some drugs, controlling the amount that enters the bloodstream. Drug companies have to increase the dosage of some tablets to compensate for the action of the enzyme.