Expert: Dr Ian Jackson - please note UK based - 2/20/2009

Question
Hello:
My name is Fran and I live in the Northeast United States.  I fully understand that anything you say is just a basic answer and that I should consult my own Physician for further questions.  I am 42 years old and have Multiple Sclerosis.  I have had it for the past 12 years and it has gotten slowly worse. Anyway, about 6 years ago my M.D. put me on Oxycontin for pain.  I was taking one 80mg pill three times a day.  It worked fine.  About 5 months ago for no apparent reason, he switched me to a new medication called Opana (oxymorphone). I take one 40mg pill 3 - 4 times a day as needed. This medication seems to work almost as good as the oxycontin but here is my real question.  I have done some research on the internet and I know that you can't believe everything you read on the internet so here goes...I read that Opana ER's bioavailability is only 10%, which means out of every 10mgs I ingest, my body only ends up absorbing about 4mg's.  So, if I am taking one 40mg Opana three times a day I am really only getting 48mg's total throughout the day.  Does that sound correct? Again, I am not questioning my Physician, and just came across this within the past few days and became curious.
Thanks for your input. (if you can)
Francesca


Answer
HI Fran

I'll start by saying that if the drug is working for you on the dose you are on then that is all that is important. The bioavailability of these type of drugs depends largely on what is called first pass metabolism. The drug is largely metabolised by enzymes in the gut wall and the liver as it is absorbed and heads to the rest of your circulation via the liver.
This is the same for other opioid type drugs e.g. oxycontin.
So you absorb a large amount of each 10 mg you take but a fair proportion will be metabolised before it gets to where it works - hence your 10% figure.
However you have to also remember this doesn't take into account the effect from the metabolites - some of which will still have an active effect.
So complex area but one not to worry about much - the oral dosage is adjusted upwards to help deal with this metabolism.
Kind regards
Dr Ian Jackson