I am a Registered dental assistant and have been exposed to many products in the field. This product is so high in percentage of Carbamide Peroxide that it works very well. However if it is used improperly, it does have great potential for irritating the gums and for creating and/or exacerbating tooth sensitivity. This will not happen if you keep it off of the gums and only use it every other day or waiting even longer in between re-applications. I use it by extruding a very small dot on to a q-tip and applying it directly on to the tooth surfaces. When I do this I make sure to keep my lips in a wide smiling position as to not get it on the inside of my lips or gums. I did not boil the trays and keep them the way they came. The reason for this is because once I apply the gel to the teeth, I place the trays in and they help keep my lips away from the gel much better than if I had done the heat forming. I also place a bunched-up paper towel inside my mouth in between the top and bottom trays while the solution is on the teeth. This helps keep my saliva from spreading the gel around my mouth and from contacting other surfaces of gum and tissue. Sometimes I need to replace the paper towel if it becomes too saturated during the process. When I leave the trays the way they come, they seem to keep my lips at a farther distance from the teeth surfaces and don't push the gel around and onto my gums. I have had some sensitivity but this is not abnormal when using whitening solutions at such high percentage of peroxide such as this product has. That being said, I LOVE the results. I saw a difference of at least 8 shades within 3 usages! That's pretty good. I did have my teeth professionally whitened originally and I do notice in my 16 years as a registered dental assistant that it seems to speed up the touch up process when this is done at some time beforehand. If the stains on your teeth are due to natural pigment or from some sort of internal discoloration then it is very likely or at least possible that external whitening will not be successful. The price is so reasonable its definitely worth a try if you want to give it a go. I am totally pleased and happy with my results and with the product itself. It is what it says it is and for me, it does what it says it does! I use such a little amount it will last me months. I will likely store it in the fridge to ensure it's efficacy for as long as possible. I say it's a great product!TIP: When I am done applying the gel to the q-tip I pull back on the syringe plunger and it draws back any gel into the syringe barrel so that none is wasted. It will suck back into the syringe if you do this.