Coffee, tea, wine, and many of our favorite beverages contains compounds known as tannins—plant substances that stick to teeth.
 

Tannins are not harmful to your health; however, tannins make your teeth look yellow when not removed after coffee. Source: Sarasota Dentistry

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Teeth that have already been stained with coffee usually do not get whiter on their own without more traditional teeth whitening treatments, which can be expensive and painful.