I agree with Gomrath.
Generally medically unfit boils down to either risk of loss of consciousness, or incapacitation. In the case of loss of consciousness, an alarmed instructor can probably handle things just fine.
Incapacitation, on the other hand, introduces risks in the cockpit, which the other pilot should be aware of, and agree to. There is special training for pilots for this risk, though I very much doubt that many instructors receive it. I have dealt with this before, and satisfied myself that a snug lap belt, and shoulder harness would keep things safe, and never had a problem anyway.
Let the instructor know if there is an increased risk, but otherwise, good on you, go keep current!