Thats a standard first line medication for chrons and UC, dont worry about it, i have been on it for 18 years and it hasnt done me any damage, only helped! It got to a stage where the pentasa couldnt keep me in remission so they had to add in azathioprine, but up until then the pentasa was great, altho they had to slowly increase my dose, i am now on 8x500mg tabs a day. Havent noticed any side effects, only get those with the azathioprine i am on or the steriods if i flare. Try not to worry, it is the first line of defence and will help you stay well for longer periods of time! Apart from the tablets being the size of horse pills they will do you well!
Yvonne xx