As it turns out, I was being over-optimistic about the pace of recovery from the side effects of the radiation therapy. What I subsequently learned was that, post-treatment, I continued to "cook" for awhile, and that the effects take three to four weeks to fade completely.
So, meanwhile, the rib/back pain continues on some level. This is being managed using oxycodone, in two forms. I get low-dose, 12-hour Oxycontin to provide a base, with so-called "breakthrough" relief provided when needed by Percocet, which is another relatively low-dose mixture of oxycodone and acetaminophen. I try to avoid using the latter as much as possible, owing to its rather dramatic push of the digestive tract towards constipation, but it is very nice to have available when I do need it.
No, there are no "highs" involved here; this is all quite low-dose biochemistry.