You need to first establish your credibility in the company and demonstrate that you are a good worker. Once that is accomplished, you may want to tell them of your medical condition.
I personally would wait till I have the job.
I'm not sure if you meant wait until you were working there before telling them but I'd strongly advise against that - in my opinion they would feel - rightfully - deceived if you witheld something important. Although I'm no expert, if you were asked in a form or in the interview and did not disclose it that may be grounds for dismissal.
Personally, I'd get it out in the open if it might have a bearing on the job and as an employer would respect someone for doing so