Legally speaking, you can treat any patient you want, for anything you want, provided you have a valid and unrestricted license in the state you're based in.
However, in the real world, you'd find it really hard to get hospital priviliges, insurance coverage and/or patient referrals without a pertinent board certification. Nobody is interested in having physicians treating patients for medical problems which they aren't qualified to treat.
However, there ARE exceptions. Some (usually highly qualified foreign docs in well-established academic centers) can practise without, or with an unrelated, board certification, but it's an exception, not a rule.
Am in complete agreement with ny skindoc. However, there's certain gray areas. Not only can and do som FP's provide derm services, but there's also dermatologists who provide services normally provided by plastic surgeons.
I am not a lawyer, so I don't specifically know if the title "dermatologist" is protected (like M.D. is), but it would certainly generally imply, and be understood as, somebody who has passed the derm boards. Thus, using the title probably wouldn't fly. However, some get around that by using "cosmetic dermatology consultant" or other such fudging (like "cosmetic plastic surgeon") and with a board certification from some post office box which only needs your credit card numbers.
Bottom line, of course, is that anyone should only provide medical services in areas where they feel competent, regardless - but of course respecting - legal issues. And by extension, one should be really careful about actively promoting, or implying, qualifications which hasn't been approved by a competent body.
