Hi. I have had the same problem for 2 years. I just recently recovered. I have an article I wrote on my blog Seasons_PostCOvidJourney.com. This site is about my journey with post covid and all the symptoms and issues I had and how I have either recovered or in treatment for issue. Post Nasal drip is natural response in your system to keep your throat and nasal passages wet. Normally, you don't notice it. However, when we start to cough or get allergies, our throat becomes irritated and our body signals to produce more mucus. Then we cough more, our throats get saturated with mucus as well as our esophagus, and we want to cough more and it becomes a vicious cycle. What we need to do is break the cycle. But how? Well it depends on what is causing the issue. If it an infection in the sinuses or Eustachian tubes (ears) then you need an antibiotic. A DECONGESTANT ONLY would also be advantageous. Do not use an expectorant unless your chest is congested. If it is allergies, then a doctor might also add an inhaler or nebulizer. I have used all. What I found most helpful and stopped both my cough and my nasal drip was just a decongestant SUDAFED PE. My doctor suggested it. If you are coughing and it is dry (no mucus) ask the doctor for Tessalon Perles. This is a pill that works on the smooth tissues of the throat and esophagus and will stop the cough. This is stop the continued irritation. I assume you have been to the doctor to see what is causing the post nasal drip. If the doctor is putting you off and telling you he does not know it or is saying it is allergies, demand an antibiotic to at least alleviate the infection if there is one. Mucinex was given to me in the hospital because I had pneumonia. If your issues are NOT stemmed from the lungs, Mucinex will not do you any good. It has an expectorant that is designed to soften and liquify the mucus and expel it from the body.
Good luck.