In the US many schools structure things this way (taking an 8 year schedule as an average)
before year 1, and summer between years 1 and 2: rotate in a lab of interest
years 1-2 : first 2 years of medical school (often the basic science courses)
years 3-6: PhD, defense, etc
years 7-8: clerkships, sub I, out-rotations, interviews, etc.
although there are some exceptions, and some flexibility (i know that here in utah I could potentially enter my PhD after the 1rst year of med school, but most people progress to the PhD after the second year. also, if i were really on my game i could take some graduate school courses while I take med school courses. Im sure that there are some people with enough brains and time to handle that, but i am not one of them! sometimes, though, as we do here, certain taking med school classes exempts a dual degree student from taken that class again in grad schoo. schools vary in this vein, though.
in the above schedule research time and "medical" time are pretty well divided by the program itself, which takes a lot of the pressure off a student. If as student is really motivated and on top of things, people often do some research while they take med classes, but again, that is not everyone.
there is a young woman in my med school class (we are in the first year) who actually has a phD and did a post doc. she is not continuing with her research right now, i think she is excited about the new type of learning and practical skills that she is learning and doing.
once out of med school most MD/PhD folks do continue on to residency and fellowship. residency does not always provide the extra free time for research that people like you may want, so that might be a consideration.
i guess if i were you (from what you have said so far) i would ask myself what i think that i would get out of medical school that would contribute significantly to my research that i can't get, or am not getting, from a PhD program alone. there are many many PhDs who do fabulous medically related research without an MD, and likewise there are many great MD's who do good research without a PhD. In order to really enjoy the time investment (and money investment, if you were to go to school in the US, that is not a futile consideration!) you might just want to be sure that it is really worth doing -- that there is something in med school that cannot be gotten anywhere else.
i dont know if that helps. i hope so!
