If you really want to do medicine, do medicine. You will get more exposure to, more experience of and more knowledge about the various career options whilst doing it. You can then decide if you want to leave clinical medicine to do something with maths. If you don't really want to do medicine, do something else.
Medicine doesn't involve a lot of maths routinely, it involves being good with figures and being able to do simple calculations quickly but very little of the maths past GCSE level. There are some specialities where a reasonable grasp of statistics would come in handy, public health and some research fields spring to mind otherwise you need little more than the basics to understand evidence.