See the chapter called Mucous as Phlegm is an excessive buildup of mucous.

Here is the definition: Phlegm is a liquid secreted by the mucous membranes of mammals. Its definition is limited to the mucus produced by the respiratory system, excluding that from the nasal passages, and particularly that which is expelled by coughing. Phlegm is in essence a water-based gel consisting of glycoproteins, immunoglobulins, lipids and other substances. Its composition varies depending on climate, genetics, and state of the immune system. Its color can vary from transparent to pale or dark yellow and green, from light to dark brown, and even to dark grey depending on the constituents.

An ME owners experience with phlegm: “We have the thick phlegm a lot also. Our vet suggested Guaifenesen, which is the expectorant in Mucinex. You can buy children’s liquid Mucinex (which unfortunately comes in ridiculous grape or cherry flavors but at least you can administer with a syringe) OR we have also tried the Mucolyxir, which you can buy on line… is made from salmon DNA and is supposed to thin the secretions…some on the list have had good results with it….we just started it so I can’t say that I have had good results yet. The Mucinex we gave when xxxxxx had pneumonia and it seemed to help her release some of the lung gunk better. HOWEVER, if your vet says that the problem is due to irritation in the throat, I would certainly consider sucralfate. When xxxxxx has any irritation at all, a quick syringe of sucralfate makes her feel better within 10 minutes. I noticed that you do not have it on your meds list…..for us, it is the one med we cannot do without. (It is not really a drug per se, but a sort of liquid or gel “bandage” that coats and helps the throat and esophagus heal and feel better….we take it as often as 3x a day if needed and she has never had any ill effects from it….for us, it is a “wonder drug”.”