Grapefruit juice is the juice of the grapefruit (Citrus x paradisi), a hybrid of the Jamaican sweet orange and the pomelo that is believed to have originated on the island of Barbados. Like most citrus fruits, the grapefruit is segmented and has acidic juice.
The main commercial cultivars of grapefruit are the traditional white grapefruit, which has yellow-green skin and off white to yellow flesh. In 1929, a patent was taken out for the Ruby Red grapefruit, which has a orange-yellow rind and pink to red flesh. Both the white and Ruby Red grapefruit are about the size of a softball.
The juice of the white grapefruit is acidic, with a bitter, pithy note to it from terpenes, a class of chemicals responsible for the green notes of juniper and the litchi note of Muscat grapes (and the kerosene secondary Riesling flavor profile). Ruby Red juice is softer, with a distinct honeyed flavor and a strong red berry fruit note. Grapefruit juice has an average pH of 3 to 3.3, which makes it less acidic than lime or lemon juice.
Health risk. Grapefruit juice has chemicals in it (namely naringin and bergamottin) that inhibit CYP3A4, a vital metabolic pathway found in the small intestine and the liver. This enzymatic pathway is responsible for the removal of a wide range of prescription medications from the body, and grapefruit's inhibition of that pathway means drug overdoses are possible after consuming merely 4 ounces of grapefruit. Some of the classes of drugs that are affected this way include antiarrhythmics (quinidine, carvedolol), benzodiazepines (diazepam, alprazolam), statins (atorvastatin, lovastatin), the big 3 male impotence drugs (Viagra, Levitra, Cialis), and many members of the codeine family (Oxycodone, Hydrocodeine). Prescription drug users should consult their doctor or pharmacist. Wikipedia on grapefruit drug interactions. NIH on grapefruit interaction with oxycodone.