It's reasonably safe to plug an oral solution. It's as safe as plugging
dissolved tablets. It's not "safe" in an addictive and/or overdose sense, but it's safe in a "not going to destroy your rectum" sense.
Plugging things generally increases bioavailability; in the case of morphine
, it GREATLY increases the effects one gets compared to oral administration. So, start slow & low. You can get an "oral" or "baby" or "medicine" syringe at drugstores & pharmacies, they're used for giving babies liquid medicine - these are great for booty-bumping anything you might want to booty-bump.
If unavailable, a turkey baster or nasal syringe/aspirator will also work.
If one already has a tolerance of some kind to oral morphine, try a 1/4th of the dose needed orally if plugged. With no tolerance it's harder to gauge, but perhaps whatever the equivalent amount of syrup that equals 5mg of morphine (the bottle should say XXmg morphine/XXmL - like, 10mg morphine/5mL syrup, which means there's 2mg of morphine in every milliliter of syrup. So just do the maths on whatever the bottle actually says, and go from there. The oral syringe often has mL marks on them, for easier measuring.
Good luck & stay as safe as possible!