I have used this product off and on for many years. Use as directed for a week, to two weeks, depending on the amount of whitness you need, it really works. I use to get it at my dentist office, they started charging alot for it so I found it on-line and have been very very happy with it. It is so much cheaper to buy on-line. I have used the Crest Strips and it is no where near as effective as the Day White.
You should initially check with your dentist before you buy Day White because your gums may not be in good enough condition to use it.
ALSO, you will need to have a form-fitted tray to closely fit your teeth to squirt the bleach into. Your Dentist can make a mold for you, or, you can try to find a snug-fitting rubber teeth tray. But if the tray is too big you will waste a lot of the bleach. You don't want that. The bleach is strong enough to blister a gum, so TRY NOT to apply it directly on the gum line. If you have healthy gums and teeth, you should have no problems. Follow their enclosed directions.
You may need to re-apply Day White in a couple years, depending on how much coffee, tea, smoking, etc. you do.
Over all this product works excellent.
Good Luck and Keep up the beautiful Smiling!