i found a post from a few months ago that was going over the same thing..........here it is and may explain the reason immodium works....

"The active ingredient in Immodium-AD is Loperamide, which is either a mild opiate or chemically related/analogous to the opiates. Plus it is anti-diahrreal (sorry) - which will help during w/d's - because hydro/codeine are very constapating so you are in for a treat (not) when you stop taking them suddenly. So taking immodium helps with this bowel problem and also even puts a mild opioid-type med into your system. They use Loperamide in Immodium *because* it is related to the opiates, which are some of the best anti-Diahhreal chemicals out there"

i'll keep looking for the graduate student's response.