I want to join an online dating service, but I have a health concern...details provided?

You do NOT want to post it on your profile. That will literally scream "MOVE ALONG!" to anyone who is even remotely interested in you prior to reading that sentence. You can bring it up when you get to know them better. No point putting a huge flashing sign on your head when you don't even know if it's a good match. Then when dates progress and sex hasn't been had yet, bring it up when you feel comfortable enough to (as you are required by law to tell). But there's no point shooting yourself in the foot by announcing it on your profile. It's a need to know on a case by case basis.