I bought a pack of these while I was in Canada - 40 strips to be used twice a day for a total of an hour.  I used these strips for three days and they gave me really bad tooth sensitivity.  The last time I tried them the pain was so bad that it woke me up in the middle of the night - I have not been ble to use them since.  Because of the short time I used them I cannot say if they work or not.