What is a good medicine for phlegm?

I'm not sure if having rhinitis is related.

During the beginning and end of a cold, there is always a lot of phlegm.

Mucus inside the throat is stubbornly stuck, neither coming out nor being swallowed, and continues to persist.

I tried various types such as syrup, pills, and lozenges at the pharmacy, but I didn't see much effect.

Even if I go to the ENT clinic and get a prescription, it's useless.

When you have phlegm that doesn't resolve even after taking medication and only slightly improves

What medication are you taking?