To summarize a bit what fledarmus: said, "Pharmaceutical" or "Food Grade" reagents have been determined to be free of things that could injure a living thing, or a human if ingested. It's safe to use as a Doctor prescribes. If, from the same chemical supplier, for found "chemically pure" or "analytical grade" and you saw the % purity listed on the bottle was comparable or better, that does not mean that its certified safe to consume. The traces of impurities may very well be dangerous.
I remember once, for an experiment, we needed a large amount of citric acid, for a dialysis. Food grade was cheaper in quantity, so we went with that. The stuff was visibly brown in color, and had, what looked to me, like wood chips in it. Still, food grade, so "yum."