Hello,
Various causes for chronic post nasal drip are allergies,viral infections,
deviated nasal septum,chronic sinusitis,adenoid enlargement
and diseases like sarcoidosis and GERD.
Pls check out with an ENT specialist if you are having chronic rhinitis or sinusitis or deviated nasal eptum. If you have chronic sinusitis then it can lead to continuous production of mucus and less drainage leading to post nasal drip.Transillumination test and X-Ray or CT sinuses can help in diagnosing it.