Tolerance to pain meds

Hello,
I have noticed over the years that the usefulness of my rescue meds
has worn off and the dosage has had to be increased.

I can remember about 10 years ago when I had my wisdom teeth out and
one Percocet knocking me out cold. Although I take maybe one pain pill
(a Lortab 10/500) every two weeks for a migraine, the same kind of
"wisdom teeth" dose helps a bit with the pain, but certainly does not
knock me out anymore.

I could understand if I was taking these meds daily or on a regular
basis, but it seems s****adic enough that they should still be
effective.

Does anyone know what is happening chemistry-wise with the body to
make the medicine less effective over time?

Is there a period from which I should completely refrain from taking
it so that I could get back to a lower dose that would still have a
painkilling effect?

Marie