Medical durations for international flight

Now that a 3rd class medical is good in the states for 5 years I was
thinking I could save some trips to the Dr. However, since I fly in
Canada occasionally and Mexico several times a year I'm wondering how
I would know if my medical would be accepted after 5 years. I
understand that info in Mexico is very slow but have we heard anything
from Transport Canada? Does ICAO say anything about medicals or does
it just refer back to the certificate issuing country for that type of
stuff?

-Robert