I looked it up and antihistamines (Zyrtec, Claritin, Benadryl, et al) can lead to weight gain.
Both water retention and fat. How so fat? I can testify Claritin causes intense carb craving which I indulged enough to gain about 7-8 pounds in fairly short order.
Forget about serving size -- have a whole bag of carby snacks.
The point of posting this on an APC forum is that I also think that ADT and being low on T makes one even more sensitive to antihistamine cravings.
Awareness of the interaction is half the battle.