Grapefruit(Citrus paradisi) is a tropical or semitropical evergreen plant cultivated for its edible fruit. Grapefruit juice is the fruit juice from grapefruits. Concomitant intake with grapefruit juice increases the concentrations of many drugs in humans. It blocks special enzymes in the wall of the small intestine that actually destroys many medications and prevents their absorption into the body. Grapefruit juice can be dangerous for people on certain medications. A glass of grapefruit juice may improve intestinal absorption of coenzyme Q10 (CoQ10) by almost 50 per cent, says a new study from Japan. The Grapefruit Juice Diet has a relatively long history, making it one of the oldest fad diets around.