I also have been wondering the same thing, since taking Allegra, I have gained 7-9 pounds. since going off, so far I have lost 3, but wanting to lose more. I take Claritin now, and seem to be ok. but i was hoping somone could answer this question too.
I have searched the internet to find out, to no avail.
I can't speak for anyone else, but I have taken Allegra-D for months and I haven't gained any weight from it. Usually, they give you out a sheet of information on the drug when you get a prescription that lists the common side effects. I don't remember that being one offhand, but I don't have the sheet anymore to recheck.
Good luck and hope Allegra works for you.
I've noticed people with allergies tend to eat alot more sugar and highly fatty foods because they can't taste or smell very well. Cravings can be incredibly subconcious. I'm on Allergra D. I am aware that because of my allergies I'm going to crave foods high in saturated fat and calories. So, some advice would be to use food as medication right now: lots of vitamin A, C, and E.