I believe that Crest white strips are Hydrogen Peroxide and usually in office whitening contains carbamide peroxide at a higher percentage.
Professional gel is a higher concentration than Crest Whitestrips. Professional whitening and the use of a custom tray leads to full coverage of the gel on the teeth with less chance of gingival chemical burns. After the initial cost of custom trays, professional gel tends to be cheaper than Crest Whitestrips too!
Check out Opalescence Go. It is the professional alternative to over-the-counter whitening options. Our patients really like it.
I believe that the ingredients are the same but the concentration is significantly lower in the whitestrips than what we would use in office.