Hi, In case of a short break (medical leave), there is no need to mention it in the experience letter. However, if it is a prolonged period, then you can.
This is to certify that Mr./Ms. [Name] was employed with us from [start date] to [end date] and from [start date] to [end date] with a break of [duration] months due to medical reasons.
During the above period, his/her performance was found to be good.