I've bought 35% Opalescence many times from my dentist and never had a problem. This stuff was awful, it's the old packaging and out-dated. It instantly started to burn my gums the moment I tried it.( almost felt like it was bubbling on my gums like acid) I took it to my dentist and he said that it's old and I need to trash it.
Also, they advertise the newer packaging in the ad, but I was mailed the older packaging from many years ago. I'm not sure how these people get a hold of a older product and then peddle it through Amazon like this? I don't write reviews very often, but I really felt the need to warn people about this stuff. Good luck.