Is tooth whitening at salon safe?

Salons who offer tooth whitening seem to be on the rise. I had mine done last year and the results weren't dramatic, in fact it barely looked any different. When I went to my dentist for my regular check up he could tell my teeth had been whitened and seemed annoyed that I'd gone to a salon. I was slightly worried by his reaction. I'm thinking of trying to get it done at another salon to see if the results are better this time but is it safe?