Studies have shown that drinking cow’s milk does not stimulate the production of respiratory mucus or obstruct bronchial airflow. In one study, 60 volunteers were exposed to a respiratory virus. They had to record their daily respiratory symptoms for 10 days. Nasal secretion weights and milk and dairy product intake were also recorded. No statistically significant association was found between milk intake and symptoms of mucus production or cough (1). Other studies have demonstrated similar results (2, 3, 4). A recent review concluded, "there is no evidence (and indeed evidence to the contrary) that milk leads to excessive mucus secretion" (5).
People who complain about milk and mucus may not consume enough fluids during the course of the day. This is especially true of older people. It is recommended that we all get 9 to 13 cups of fluid a day. Water, fruit juice, milk, yogurt and soup are a few choices that count for fluid.
(5) Archives of Disease in Childhood, Vol. 104(1), 2018. doi.org/10.1136/archdischild-2018-314896