Your body does get used to the doseage. I did amitriptyline for a while, starting at a low dose, and tried to build to 4 times that amount. I got "stuck" at the 3x level (I cannot for the life of me remember the doseage). Initially, I was super dizzy and out of it after taking it, and after the second time, started taking it just before my head hit the pillow, 'cause I couldn't even brush my teeth the 2nd night. I would have had to be careful if I'd waken in the middle of the night, but I didn't because I was knocked out. I could not hardly function the first few mornings I took it, and the doc had told me to expect that, so I started up on my days off. I definitely slept, and my back definitely "relaxed" (which was the "target")... However, one of the side-effects is difficulty urinating, and that happened at about the four week mark, and got worse daily. I quit for a few weeks, and started back up (with doctor's monitoring), and my body "remembered" where I was with that side-effect - really strange. I quit again and haven't used it since. I do wish there was something like it that could take its place, without the side-effects... sigh.
"There but for the grace of God, go I."
"... His mercy endureth for ever."