I had a colonoscopy in 2003 and had to drink a gallon of that awful laxative (don't know the name of it) and that is actually what caused my current fissure problem.
However, I just had another colonoscopy in December 2006, and it was amazingly easy and didn't bother my fissure much at all - a miracle! The medical community has a new laxative for colonoscopies that is so much better (I would insist on this one)- there's not nearly as much to drink and so you don't have to visit the toilet as often, thereby lessening the damage to an exisiting fissure and lessening the possibility of causing a new one.
I guess I would say my fissure was slightly worse after the laxative, but not a lot. And, the procedure itself didn't bother it at all. Be sure to mention your fissure to the surgeon on the day of the procedure so it will be fresh in his/her mind.
By the way, colonoscopies are EASY. Frankly, my husband and I both enjoyed the anesthesia.
Good luck to you.