I agree in general with the previous answer, however I would not normally refer to the content as either ingredients or composition in normal conversation. What I mean is I would say, "I went to the doctor and te gave me some antibiotics. (tablet or liquid)" so the active ingedient almost becomes an adjective to describe the medicine.

Ingredients, or sometimes 'active ingredients', as they don't usually list all the 'fillers'. Yes, a liquid medicine is still medicine, but we would probably say a bottle, rather than a jar - jars are more for pastes. I would probably say packet of tablets, but box is fine too.

I think, In addition to Gary's answer, composition is a term used more for science based information. Like the composition of elements that make up something.Composition is also used in describing art and music. Ingredients is more appropriately used for everyday language usage like in foods, or medicine.