Can dentures cause a phlegmy cough? If so, what can be done? Mom has had a bad, phlegmy cough for a month that seems more when wearing dentures.

Tkd123, for myself, I found dairy products would cause a coughing phlegm type problem.

Oh, dust also caused a similar issue. Dust likes to hide on the carpet/floor behind the headboard of the bed.

My 103-yr old Aunt has a phlegmy reaction sometimes when she eats lunch or dinner -- to the point we have to have a pail next to her so she can clear it before continuing to eat. It is a lot of phlegm and it exhausts her. We've started to give her Mucinex and it seems to help BUT I wouldn't do in your Mom's case without consulting her doctor and discounting the other serious issues already mentioned by others. It could also be a gag reflex from her now ill-fitting dentures...

Yes at this stage I don't think its the dentures. Your Mom may not be able to swallow anymore. Need to get her to a doctor for a swallow test. It could be she is transitioning. Hospice may need to be brought in.

Do you notice the cough more when she is eating or a short time later? If so it may not be the dentures but she may be aspirating food and fluids.
I would consult her doctor it is possible that her food needs to be pureed and liquids thickened.