What are antagonistic drugs?

Drugs that bind to the receptor site and inhibit its natural function are called antagonists, drugs e.g., cimetidine is an antagonist drug because it binds to the receptor site in the stomach otherwise histamine will bind to the receptor site and induce the secretion of HCl in the stomach.

Thus, this antacid blocks the receptor site and hence, the secretion of HCI is reduced thereby reducing/removing hyperacidity.