Need help. i've got a question, are tooth whitening strips bad for you?

No: They are not 'bad', but they require good technique and consistent use to get the results in commercials.

Bleaching teeth: Crest white strips can work, though they only cover the front 6 teeth, so if there is a big color change, it may be very noticeable compared to the back teeth. Avoid using any product that requires a "pre rinse" as this is usually acidic and can destroy tooth structure as well as make them more sensitive. A kit custom made by a dentist is the most effective method.