Post nasal drip causes a sore throat because the excess mucus that drips down the back of the throat irritates and inflames the sensitive tissues there. This constant contact with mucus can lead to swelling and soreness in the throat, including the tonsils. The mucus often contains inflammatory substances that exacerbate this irritation. Additionally, frequent throat clearing triggered by the feeling of mucus dripping can strain the throat muscles, further contributing to the soreness. If the post nasal drip is due to an infection, the infectious agents themselves can also inflame and irritate the throat, making pain worse.

Key factors why post nasal drip causes a sore throat:

• Prolonged exposure to thick or excessive mucus irritates throat tissues causing inflammation and soreness.
• Mucus may contain inflammatory substances from infections or allergies that worsen irritation.
• Frequent throat clearing due to mucus presence strains throat muscles, adding to discomfort.
• Infection-related post nasal drip can directly inflame the throat tissues.

Thus, the sore throat associated with post nasal drip is primarily due to inflammation and irritation from mucus and possible infection or allergens contributing to it.