I've heard a few people talk about that here in Portugal, and supposedly an american medical degree does not grant immediate equivalence in Europe (the same goes for an european degree in the US). I think applicants need to take an exam (though I don't exactly know what kind) in order to be authorized to practice. They also need to be fluent in the language of the chosen country. There must be some variations from country to country, but I assume the general rules are mostly the same, at least inside the EU.
Generally, embassies and consulates can give you this sort of information. You could also try to contact the national medical organization or the specialty college of the country you're interested in.
