Hello and thank you for your question.
A tooth whitener which is generally hydrogen peroxide
or carbamide peroxide
reacts with surface stain gently removing them from the outer surface of your teeth. These products may cause an irritation or bumps on the outside of the mouth. If this occurs you may have a sensitivity to the product. Make sure that you don't have any lay on the surface of your skin and keep it on your teeth. Pimples
are products of bacterial infections in the pores of your skin. Since these products do not contain pimple causing bacteria, it is unlikely that they would cause these issues.
Irritation, tooth sensitivity to cold
, and gingival soreness and signs that too much has been used and have been in contact with other places other than your teeth. Remember that tooth sensitivity caused by whitening agents is temporary and can be relieved with a anti sensitive tooth paste.