But when you have a cold, congestion, or allergies, the amount of mucus can increase, build up, and thicken. The feeling of this mucus draining down the back of your throat is postnasal drip. It often causes a sore throat, coughing, and problems swallowing. See a picture of postnasal drip . The increase in mucus that leads to postnasal drip can be caused by a cold, flu, allergies, sinus infections, or hormonal changes. Gargle with warm salt water to help get rid of throat soreness. Stir 1 tsp (5 g) of salt into 8 fl oz (240 mL) of lukewarm water. Gargle as often as you like-the more often, the better.