It is the job of marketing to get your attention and convince you to go to the store (or the pharmacy). To do this, marketing content needs to make you believe a product is worth your time–that the amoxicillin will cure what ails you. Marketing is about affecting opinion, how people feel about a particular product.
Merchandising, on the other hand, delivers the goods. It aims to send you home with a product–the antibiotics, in this example.