Please help. What would be the difference from:
1. getting MD, and then, doing research without PhD along while practicing medicine?
2. getting MD/PhD and continuing research and practicing medicine?
1 and 2... Practically none once an attending. You have to train at some time for approximately equal amounts of time. Whether you spend those years training during med school/grad school, during residency, or after residency is up to you.
Is it more difficult to be accepted into the MD/PhD program? What are the additional requirements and application processes for the MD/PhD program? Is it possible to be accepted into the MD program, and then, switch to the MD/PhD track?
At most schools the MD/PhD program is harder to get into than the MD program, as the stats are typically higher, state residency is not taken into account for MD/PhD admissions, and extensive research is required in addition to the clinical experience required for MD admissions.
For requirements and application processes, do a google search. It's essentially the same as med school + extensive research experience (2 years or more, generally) + extra essays and interviews. See the sticky guide in this forum for general competitiveness information.
It is possible to be accepted to the MD program and then switch to the MD/PhD track. How common or difficult this is depends on the individual medical school, and that ranges anywhere from it never happens to almost everyone who applies gets a MD/PhD spot. Generally, they will look at your application as:
1) Is this person someone we would have accepted had they applied MD/PhD in the first place?
2) Is this person performing well in med school?
3) Is this person continuing research in med school OR shows enough promise before med school that we can ignore this.
If you are accepted MD/PhD from the MD program you will have to pay for the first one or two years of med school. Thus, I don't recommend this pathway for anyone except the rare person who really couldn't decide before they applied but suddenly decides during the first or second year that they need a PhD for their personal goals.