I had ZOOM whitening 2 weeks ago. Before starting, the dentist explined that once in awhile a patient will have pain, some lasting a day, some lasting several days. Also, he added that if there are problem teeth, expecially near the front, there can be additional problems. I had BriteSmile several years ago and had no problems at all, so I was pretty sure things would be OK. I told him to go ahead. I did have some pain for about 1 1/2 days. Over all I am thrilled by the results. I really think a lot of it is your pain tolerance and the condition of your teeth.