It depends. Was the payment for the medical expense the same year you received the refund? If so, there's nothing to report. Just remember to decrease your medical expenses by the refund if you are itemizing and claiming medical expenses in excess of 7.5% of your AGI.
If the refund was from a prior year, but you didn't deduct any medical expenses on Schedule A, again, there is nothing you need to do. A refund of a personal expense is not taxable income.
However, if you received a refund of a medical expense for a prior year that you DID take an itemized deduction for, the best course of action is to amend the return and decrease the medical expenses by the refund. Here is how to file an amended return: How do I amend my federal tax return for a prior year?