Dentist gave me a free kit. First (only) time I tried it I had to spit it out after about 12 minutes because I was drooling so badly and the little slippery gel holders were slipping off. I read all the reviews and nobody mentioned this problem. I wonder what I did wrong!
Most people would salivate during teeth whitening because of the flavour and even acidity (in some cases) of the gel. You can spit it out. You eventually get used to the process and salivate less. If your trays come out too easily then they aren't fitting very well. The best ones are custom made for you by your dentist.