From what I understood, in addition to the information from hopinso above, the citrus juice in effect takes the place of the stomach acids, so that the time the oil spends in the stomach is much less. Otherwise, the oil would be sloshed around for much longer, and the stomach's job much harder, in having to secrete the chemicals and to mix them with the oil in order to send it past the stomach. Having that much oil in the stomach alone for that long, without the citric acid , would be far more uncomfortable, leading to much more nausea and likelihood of vomiting. However, the juice mixed with the oil hastens its passage into the small intestine and beyond, since the oil's job is primarily to stimulate the onslaught of bile that will push out the stones.
I'm a grapefruit juice gal myself; no real problem with drinking the oil/juice, only the nasty Epsoms!!! ;#