I have completed my MD. I am more interested in research than in clinical practice. I know there are post-doc positions available which accept post-MD candidates. Still I am thinking of doing a PhD in my specialty of interest. However I would like to know what are the benefits of pursuing a PhD after receiving an MD? Is it really necessary to obtain a Ph.D. degree for a research career?
Look at it this way; an MD says you are qualified to practice medicine, and a PhD says you are qualified to do research. In theory you could practice medicine without an MD (it'd be illegal in many countries but not having an MD does not mean you don't have medical knowledge), and likewise you could do research, especially part-time, without a PhD.
Leaving semantics aside, having that degree significantly increases your chances of pursuing a career in a particular field. If you intend on leaving the clinic and devoting your time and energy to research I'd say that a PhD would be expected, when you look for jobs. Whether or not you can bypass that expectation with other qualifications would be speculation.
As it turns out an MD & PhD combination is... uhmmm how to put it delicately.. "so money". :)
I say you put your hat in front of you, figure what you want to do, and go for it. Many doctors here (in Sweden) go for a double degree and it is definitely not something that's frowned upon. On the contrary, in many cases it is an additional merit for future promotions.
I'm an epidemiologist, working with a large number of research-oriented MDs, as well as a number of MD/MPH, MD/PhD and PhD researchers.
In terms of an MD vs. an MD/PhD...
I haven't met anyone whose been held back by doing research with just an MD, but my field admittedly somewhat exalts clinical expertise. The biggest advantage of a PhD is that you will have spent a great deal of time doing research instead of clinical practice. If you want to be an expert in methods for research and analysis, that's where a PhD comes in. The MDs I work with often end up deferring to a PhD (like me) when the research goes from the high level to genuinely in the weeds.
It is very much depend on your country and research field. Technically speaking many medical doctors affiliated to hospitals do research and publishes the results, so it is definitely not impossible. However, you should ask yourself if you have enough technical knowledge in your field you want to go and if you have enough experience in setting up research programs, writing proposals and papers etc.