When Rex was so sick Deramaxx was the only thing that helped hi,. Rimadyl and Tramadol didn't do a thing. Rex had bouts of colitis and a very iffy tummy so I was sort of fearful at first, but we never had any side effects at all. I'm not sure how much Duke weighs, but 1/2 a 100 mg sounds like a pretty low dose. Rex was pushing 90# and started out at 100 and went to 150 toward the end. Only thing to suffer ill effects was my pocketbook.