Drinking grapefruit juice together with certain medicines can increase blood levels of the drug to harmful levels, a new report claims. Researchers at the clinical pharmacology department at Radcliffe Infirmary in Oxford, England warned that grapefruit juice inhibits a chemical in the intestine needed to break down many drugs in the body. The absence of this chemical can lead to higher blood levels and the drug may become more potent. This effect has been observed in nearly all calcium channel blockers, a group of drugs used to control blood pressure. Full details are published in the January issue of the journal Nature Medicine.