I've been told that too, Matt, by pharmacists, physicians, and nurses who were expert in pain management. I've been searching for a web site that explains it well but its usually just stated as a general warning. This is the best I've found so far:
"Metabolite norpropoxyphene has 1/2 life of 30-36 hours Norpropoxyphene is the metabolite and its
accumulation with repeated doses may be responsible for some of the observed toxicity.
Norpropoxyphene the active metabolite has local anesthetic effects and like lidocaine, may cause cardiac conduction delay (increased PR interval and QRS prolongation)."
"Bioavailability reduced 30-70% due to extensive 1st pass metabolism. 80% of drug is bound to plasma proteins and stored in fatty tissues. Great variability between people in the rate of clearance and plasma concentration. Crosses the placenta."
With a drug half life of 6-12 hours, and a metabolite half life of 30-36 hours, you can see the potential for problems, especially in people who have any sort of compromised ability to clear it.