Medical science is both a vast domain of knowledge - a branch of health science aimed at diagnosing and treating human ailments - and the practical application of that knowledge in a human context. It includes virtually all the aspects related to medicine, its branches, classifications, specializations, surgery, ongoing researches, and education, including veterinary medicine and nursing.
On the other hand, a common man always expects his/her doctor to pen the prescriptions - all 'medicines' - on the notepad, which he/she will then get it from the nearest medical store. The technical meaning of 'medicine' is pushed to the backseat in this context.
So, if you hear somebody is going to get the medicine, assume he/she is talking about drugs, and if somebody tells you that his nephew is studying for medicine, think he is a doctor in the making. Quite an interesting synonym, isn't it?