Last week my dh and I had to attend a short seminar about prescription meds through his work for insurance purposes.

One thing the guy said is that if you drink grapefruit juice, it can block an enzyme that works with time-released medications (and I suppose vitamins?) that makes it release all at one time. This can be extremely dangerous, depending on the medication. Also, I also learned that drinking soda with iron will make the iron null and void.

Also, if you take antacids, they may block absorption of vitamins and medications, so it is like you are not taking any.