Leoperomide

interesting question. i'm not a chemist or a doctor, so you have to take my opinion with a grain of salt. but this drug is used to combat severe diarrhea. if that is the case, it seems to me that the drug will cause you to hold onto your bowel movements for longer than you would if you had severe diarrhea, so for that short window it would appear that you are holding onto mass. however, holding onto your bowel movement doesn't actually impede your weight loss by itself.

a side effect of loperamide use is constipation and less than 2% of those that were tested in a double blind study also reported dizziness. . weight gain was not a reported side effect. considering weight gain was not reported in people who continued to eat and drink the same as they did before taking the drug, i would surmise that weight loss is still possible while on it for people who are on a calorie restrictive diet and exercise regimen.

http://www.drugs.com/sfx/loperamide-side-effects.html

further reading (FDA fact sheet)

http://www.drugs.com/pro/loperamide.html