Grapefruit juice contains chemicals that can cause problems with enzymes that break down certain types of medicines in your body. When a medicine doesn't get broken down properly, you can have too much medicine in your blood. Having too much medicine in your blood increases your chances of having side effects.
If your usual diet includes grapefruit or grapefruit juice and you've been prescribed a medicine that's affected by grapefruit, speak to your GP or pharmacist. Do not stop taking your medicines without advice.