I think it is harmful to some if you take too much for too long. You can look online for reviews of Asacol and similar drugs and find more information. I, personally, am more worried about
being in a flare than taking a higher dose of 5asas right now but when in remission, you shouldn't take that high of a dose over a long period of time. A recommended maintenence dose is usually 2400mg a day orally, some people take a bit more, like 3600mg, some even take 4200mg but 6000mg for long term is a high dose although I am sure plenty of people take it and are fine.
I agree with Quincy, you can take the medication rectally with no harmful effects, it is topical. I find it strange when doctors don't want to prescribe enemas. I am thankful my doctor is the opposite, she is all about rectal meds.
here are some reviews for Asacol
http://www.askapatient.com/viewrating.asp?drug=19651&name=ASACOL