Adding the clinical framing, because it changes how the question reads.
Back pain as a lean mass side effect: not from the medication itself, but from rapid body composition changes. Losing 45 lbs shifts your center of gravity and your core muscles need time to adjust.
Physical therapy has been essential. Core strengthening exercises (planks, dead bugs, bird dogs) daily for 10 minutes have eliminated my lower back pain. Your body is literally relearning how to carry itself.