Saying something is effective in contrast to something making you feel better in a medical context are only slightly different. P-A (assuming that this transcript isn't of a conversation) would be more likely to be in a context surrounded by others in the medical field as "effective" is a word that is less likely to be used in bedside manner. Thus, P-B is in a less formal context, most likely with a patient saying "this medicine will make you feel better". The distraction chiefly rests on the formality of the context they're in