It depends on which PhD department you're doing the PhD portion in. If your PhD will be in the biomedical sciences, I don't think MD/PhDs take many courses on top of the med school curriculum. They might do some electives during the preclinical years that are in the basic sciences to help with their research. But if, say, your PhD is in something like chemistry, most departments will still have you satisfy their course requirements and many of your med school classes may not count. So students will still have to take some courses and if you've taken grad-level chemistry courses before, depending on where you took them, the school may recognize those as fulfilling part of the requirements.