Antacids can significantly slow the absorption of so many vitamins, minerals, and even prescription meds. It has to do with a liver enzyme that's commonly used to process all sorts of "stuff" (I'm blanking on words this morning). Antacids end up using up a significant portion of that enzyme and there's not much left to process anything else for a while until the antacid is fully metabolized and the enzyme replenished in the liver.

Don't ask me how I know that... It's not a fun answer.