is it possible to practice medicine after one year of residency in certain states? does it matter if you do the intern year in medicine, or psychiatry or something else?
Very popular question (hint: do a search and you will find tons of threads on this topic).
In short, you need only an unrestricted license to hang out your shingle and practice medicine.
An unrestricted license requires passage of Step 3 and a variable amount of post-graduate medical education; the minimum required is generally 1 year for US grads and more for FMGs/IMGs. The following link gives you info regarding the amount of GME required to obtain a license:
http://www.fsmb.org/usmle_eliinitial.html
The states do not designated what the training must be in - ie, you can be a psych resident or do some training in medicine. The licenses do not differ based on your specialty training.
And although you didn't ask, I feel compelled to tell you that should you be considering this route (ie, quitting residency after getting your license), you will be severely restricting your earning capabilities. Many hospitals, HMOs, etc. will not hire someone who is not Board Eligible (which requires completing a residency) and insurance companies are starting to balk at paying for treatment by someone who isn't BE as well.
That doesn't mean you can't do a single year of residency in Psychiatry, get a license, hang out a shingle, give Botox shots all day and make lots of dough...you can. And while most patients would prefer someone with relevant training and credentials, boutique practices are cash on the barrel without insurance requirements and whatever sucker fish you can shoot in the barrel...its all yours to keep.