Why some medical pills should be taken before or after or during meal .what's the reason behind this .

Pharmacokinetics: A good correlation exists between the pharmacologic response & plasma concentration of a drug. For oral medication, this is in part related to the effectiveness of absorption from the gut, & first-pass metabolism after the drug has entered the bloodstream from the gut, which in turn can be altered by the timing a drug is ingested. Ref. http://www.ashp.org/DocLibrary/Bookstore/P2418-Chapter1.aspx