My dentist gave me a tube of poladay CP 35% tooth whitening gel with my Invisalign trays. Is bleaching my teeth once a month OK?

My dentist said to use the gel as often as I see fit. My teeth are dramatically whiter for the first few days, then gradually lose their whiteness over the next few weeks. Is it safe to bleach my teeth once or twice a month or will there be negative side effects? I have minor sensitivity but nothing too terrible (yet). Thanks!