The side effects of phlegm in the stomach are somewhat dependent on the cause of excess production of phlegm. Illnesses such as the common cold and respiratory or sinus infections all cause an excessive production of phlegm, which often drains down the throat to the stomach. This can cause constipation, vomiting, and bad breath.
One side effect is constipation. If too much phlegm accumulates in the stomach, it can stagnate — in other words, it just sits in the stomach without moving. This can cause a blockage of the normal digestive tract, which means difficulty having a bowel movement.
Nausea and vomiting are often side effects of phlegm in the stomach. This reaction can be caused by a number of factors, but a lot of phlegm can create a heavy, nauseous feeling, which can lead to vomiting. The nausea may be made worse by an existing cold, which caused the accumulation of phlegm to begin with. Sinus infections can create a near constant flow of mucus down the throat and into the stomach, which can also trigger this reaction.
Loss of appetite is another common side effect of phlegm in the stomach, for similar reasons that cause vomiting. The fullness sensation that mucus in the stomach can create will often lead to loss of appetite. Nausea from the excess phlegm can also cause a person to not want to eat. Loss of appetite is a common cold symptom, so if a cold is responsible for the stomach phlegm, the already existing symptom may just be exacerbated.
Phlegm in the stomach also causes bad breath. Phlegm is made up of a certain protein, which is broken down by the anaerobic bacteria that lives in the stomach. When the protein is broken down, sulfur is released, often causing the breath to smell bad. This bacteria is also present in the throat and so can cause bad breath even if the phlegm has not yet reached the stomach.
In order to treat phlegm in the stomach, the sufferer has to understand what is causing the problem to begin with. Phlegm is originally produced in the nasal cavities, and when too much is present, it drains down the throat into the stomach. Treating the cold, respiratory or sinus infection that is causing the phlegm is a first step to dealing with the problem.