Summarize this article:

Some whitening strips contain hydrogen peroxide, which is poisonous if swallowed. Try to avoid swallowing any of the gel or saliva that is produced when wearing the whitening strips. Don't worry if you accidentally swallow a small amount of gel. Small amounts will likely not produce any symptoms. The chemicals in whitening strips can cause some sensitivity in the teeth and gums to occur. This may be the result of the gel coming into contact with the gums directly, applying the strips for too long or too much, or a sensitivity to the gel. Take a break from whitening strips if you notice your teeth or gums becoming sensitive. Ask your dentist if you are able to use whitening strips safely. Not all tooth whitening strips use the same chemicals and processes to achieve results. Some may actually exacerbate preexisting dental issues or cause new ones to arise. Ask your dentist if whitening strips and other whitening products are a good fit for you.

Summary:
Don't swallow any gel from the strips. Stop if sensitivity occurs. Ask your dentist before using any whitening product.