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A toothbrush is an item that can last a relatively long amount of time. Eventually, it'll however get worn out. Thus, according to the Centers for Disease Control and Prevention, a toothbrush should be replaced at least once every 3 to 4 months. Of course, everyone will have slighlty different brushing habits which can lead to the brush being worn out much faster. In these cases, just replace you're tooth brush once it looks worn out.