Review title is Zingers! [SEP] I used Zoom whitening years ago and got zingers that evening. I’ve used my retainer trays and gel bleach purchased from my dentist here and there since. I just finished my Invisalign and wanted to treat myself. I had my first treatment at my orthodontist’s office and purchased the take home kit (same as this item). I got zingers despite claims that it’s pain free. I took Tylenol promptly and have only felt a couple more, but they DO happen. Talked to a couple of others who’ve used GLO and they too got zingers. The manufacturer has been Told about this And claims these folks are “outliers.” I’d do it again because the results are great, but take Tylenol preemptively and plan to not eat or drink anything hot or cold in the first 1-2 days.