half the applications only per week and my mouth sores and swelling [SEP] i ve used this twice they thought of everything from lip gel to the neck cord so you can move around but each time after the first of the applications and only not back to back my lips swelled up and had painful sores inside my mouth i reapplied the mouth gel carefully with each use of the mouthpiece too i have never had an issue with other whiteners gong back to crest strips