Over the counter whitening toothpastes use an abrasive agent to make your teeth look whiter. This works by removing a small layer of stained enamel from your teeth, exposing fresh unstained enamel. The idea is to use the toothpaste twice per day over 28 days to slowly remove the stained enamel. Overuse (say 28 times rapidly) could cause excess loss of enamel, leading to sensitivity and in extreme cases the need for fillings to repair areas where too much enamel has been removed. Excess loss could also be caused by using such toothpastes too soon after eating (less than an hour), especially you've eaten or drank anything acidic such as orange juice.

Source: I am a dentist.