Teeth-whitening strips are thin, flexible plastic (polyethylene) strips with a thin film of hydrogen peroxide on one side. Whitening strip kits come with two types of strips: strips for the upper teeth and strips for the lower teeth. The whitening agent is applied by placing the strips across your teeth and gently pressing the strips to ensure contact with all your teeth. Teeth-whitening strips are typically worn for 30 minutes a day, twice a day. The duration of treatment will vary.