Plugging heroin works just fine, and it's not subject to first pass metabolism as the rectum's blood supply doesn't have to pass through the liver...
Just dissolve it as you would if you were going to inject it and then plug the resulting solution for maximum speed of absorbtion.
Here, rectal and oral oxycodone have same bioavailability of around 50%, and rectal has the slowest onset(and they used 30mg, compared to 9.1(odd figure) oral(don't remember IV dose, though it provided rapid relief, and double the potency(dose adjusted, presumably) although, another study gave similar results comparing just IV and rectal, though noted rectal administration lasted much longer, up to 12 hours
(This is about H, though finding trouble with figures for rectal diamorphine, however, at the bottom of this post, click the link and scroll to bottom, and read)
https://www.ncbi.nlm.nih.gov/m/pubmed/1485370/?i=3&from=/7818116/related
Rectal, oral and sublingual result in NO detectable diamorphine. This is in the detailed study below, though it focuses on IV vs "Chasing the ? dragon"
Intranasal does, and has a faster TMAX.
Partial first pass metabolism can take place.
Point is, if he were experienced, it's viable, though provides no advantage over snorting,
and is more trouble.
Am having trouble myself digging up studies with figures for rectal H, though from memory, it was more effective than rectal morphine (figures vary, though BA ~30%, and Tmax about 30-40min in study)
Though with diamorphine, it may be absorbed more efficiently, though it would simply be like rectal morphine, only improved BA%(to some extent) and faster onset.
Intranasal diamorphine isn't like injecting, though it is the closest you are going to get.
With most opioids, Rectal administration results in a delayed onset, though with a longer duration; BA varies, though is similar in commonopioids, though with a lower Cmax, much like sublingual administration, it's counterpart.
Oxymorphone is prime example of this(it lasts longer plugged, though onset delayed to some extent and similar BA, though lower plasma levels IIRC, oxycodone is similar(Tmax is extended)
http://onlinelibrary.wiley.com/doi/10.1111/j.1742-7843.2006.pto_233.x/full
