I'm no expert and come from a social anthro background, but I'll provide my opinion FWIW. I guess it depends on the program. For a decent PhD program, I would suppose that if you don't have a solid grounding in medical anthro research/lit/theory and don't have a 'research match' - then I'm guessing chances are slim.
I know that one person in my Masters program was admitted (with funding) from a non-anthro background, but she had a solid research idea/proposal, and knew all the hot/relevant theory for her 'worthwhile and creative' research topic. In certain cases, you might be better off doing a Masters first ...