Postnasal drip is when mucus drips down from the back of the nose into the throat. Many things can cause postnasal drip, including allergies, pollution, certain foods, or changes in temperature. The mucus can have different colors and thicknesses. This depends on what is causing it. A common cause of postnasal drip is swelling of the inner lining of the nose. This is called rhinitis. Sometimes, this is caused by allergies.