Yes, I Have Been On Percocet 7.5 For About 6

yes, I have been on Percocet 7.5 for about 6 years for chronic back pain, and I took 5 pills a day as ordered by dr. This summer he lowered it to 4 times daily due to a lower level on my urine test, I could not understand this as I have taken them as directed, and even with taking 4 daily I have had to take 5 occ for the pain. and now my last urine test came back with a low level again, so he is limiting me to 3 a day. Is there some reason for this as I am taking my meds as prescribed. This has happened to someone else in my family also. Thank you