Im on opioid replacement therapy (suboxone) and would like to know if miralax and/or prune juice are safe, long term, for constipation. i never get constipated off of opioids?

Yes, they are safe.

It would be best to avoid chronic laxative use. Prunes & prune juice & stool softeners would be OK. Drink lots of fluids, eat fiber in the form of fruits& vegetables. Regular exercise helps bowels to be regular. You could try a daily bowel stimulant like senokotS according to instructions. Try to take miralax only as needed