Take less opiates, make some dietary changes, be sure to always use a very smooth, lubricated syringe, be sure to use proper technique for rectal administration to minimize harm.
I don't think that it's the rectal ROA that's inducing the constipation bro, but you should probably lay off that ROA for a while. Do you have any visible issues going on besides constipation, like hemorrhoids? You'd still be constipated no matter how you administer the opiates, they are constipating, it's in their nature to stop your intestines natural muscle contractions called peristalsis, which helps move your food along through your GI as you digest it, and it delays the transit time / motility between passing stools, so your intestines keep sucking all the water out (especially if your already dehydrated), and this makes the fecal mater even more difficult to pass due to lack of moisture.
Consider adding a gentle stool softener for a while, add coconut oil to your diet, like 1 tablespoon of pure raw unrefined virgin coconut oil, and maybe a laxative like sodium docusate.
