While grapefruit juice is a healthy drink, it has been discovered that a chemical in the juice affects how certain medicines are absorbed, with the result that normal medicinal doses act like higher doses. Getting the wrong dose is dangerous. Since it is always desirable to take the lowest effective dose, the best medical approach would be to take lower doses of these medicines along with prescribed amounts of grapefruit juice.
The statement 'The amount of the chemical in grapefruit juice is highly unpredictable from glass to glass' most seriously weakens the argument.