The myth of milk causing mucus and worsening asthma symptoms is so long standing that many doctors suggest eliminating milk. In a study where people believed that milk caused mucus, they were giving both milk to drink and soy milk. They reported the same symptoms for both, believing the soy milk was cow’s milk — a case of mind over matter.
In people deliberated exposed to the common cold virus, drinking milk did not worsen their runny nose, cause more coughing or create more congestion.
For asthma suffers milk does not worsen symptoms and there is no evidence that milk drinking causes asthma. Drinking milk does not alter lung function. There are a few reported cases of people with a milk allergy who have symptoms similar to asthma and this may be the root of the asthma/milk myth.