I have a few questions about MD/Ph.D programs.
1) If you have completed, or are pursuing, or have an interest in a MD/Ph.D program, why did you choose to do so, what do you like about it, and what do you dislike about it?
Even before I applied to MD/PhD programs, I knew that my interests were a bit eclectic and unlikely to fit into a single career box. My personal interest is in pharmacology (drug design, drug effects, drug resistance, etc), which clearly spans both the medical and research arenas. Ultimately, I applied to MD/PhD programs because I knew that I needed a lot more training before I could work independently as a medicinal chemist (and pharmacokineticist, etc) and I just couldn't see myself picking up that kind of expertise during a fellowship.
Now that I'm here, I appreciate a few things: the ultimate freedom and responsibility I have for my own education, the ability to network with just about anyone in my field, and the gift of working with some seriously brilliant chemists on some really cutting-edge research. On a lighter note, I also really appreciate my ability to balance my work schedule with my martial arts training. However, I don't enjoy all of the red tape (having to interrupt work days to give throw-away grad school presentations, having to take classes on things I already learned in medical school), the uncertainty (will I really get my PhD in 3/4 years?), and the fact that I see my clinical knowledge all but disappearing as the days go by. But, you have to take the good with the bad, and I still believe it was the best choice for me.
That being said, my advice to anyone considering an MD/PhD program is to evaluate your career goals before you choose this path. If you want to conduct clinical research, or bench research that is technically-light, you may very well save yourself some time by getting just an MD and pursuing research during residency/fellowship. Only go for the PhD if you need it for some reason - it is too much time and frustration to do just for the stipend or prestige.
2)Is it possible to enter a schools MD/Ph.D program if you enrolled in MD only? if not would you have to do Ph.D separately(applying to a bunch of schools, and the interview process all over again), or are there other options?
This depends on the school. Many schools will tell you that it's possible, but in reality only a few will make a common practice of doing this. It is usually a good idea to ask current students how often internal applicants are accepted - they have no reason to lie to you.
If you want to pursue the MD and the PhD separately, you should really get the PhD first. Once you graduate with your MD, you will be expected to go directly through residency AND you will have a large amount of debt to pay off. Both of those things would complicate graduate school plans. Honestly, though, if you end up with an MD and you still want to do research, apply for research-heavy fellowships and get your training done then.
3) As an MD/Ph.D, do you still get the patient interaction that MDs/DOs do, or is it much more research based?
This depends on the person. Some MD/PhD graduates end up working exclusively with patients. Others work only in lab, and some will attempt to balance the two. How much patient interaction you have depends on your own priorities, your grants/lab demands, and the place where you end up working.