It seems since taking asacol (mesalamine) hd i've experience weight gain and accumulated belly fat despite diet and exercise. could this be caused by the med?

Unlikely : Most of the research reports quite a few side effects from Asacol (mesalamine) hd, but weight gain is not mentioned as a reported side effect. It does note, however that increased appetite and abdominal distinction can occur, but not often. Perhaps these last two possible side effects are contributing to your symptom. Might be a good idea to discuss this further with your doctor.