Grapefruit and its juice (along with Seville oranges and limes) contain chemicals that prevent enzymes in the gut from working properly, with the result that more medicine is absorbed by the body. This can result in overdoses and serious side effects.
Taking one tablet with a glass of grapefruit juice can be like like taking five or even ten with a glass of water. There are currently 43 medications (March 2013) that can react with the juice. Almost all of these are available on prescription (they include statins, blood thinning medications and painkillers), so you should receive information from your GP (and your medication's leaflet) about the risks. If you are concerned about your medication, speak to your doctor or pharmacist.
