Does Milk Cause or Thicken Phlegm?
No, milk does not cause or thicken phlegm in either healthy individuals or those with respiratory conditions including asthma and COPD—this is a persistent myth not supported by scientific evidence.
Evidence from Controlled Studies
The most definitive evidence comes from rigorous clinical trials that directly tested this widespread belief:
A 2020 randomized, double-blind, placebo-controlled trial in 96 children (both asthmatic and non-asthmatic) found absolutely no changes in respiratory symptoms, spirometry, fractional-exhaled nitric oxide (FeNO), or oxygen saturation at any time point up to 120 minutes after cow's milk consumption compared to soy milk substitute 1.
A 1990 study of 60 adults challenged with rhinovirus-2 and followed for 510 person-days found no association between milk intake (ranging from 0-11 glasses daily) and nasal secretion weight, upper respiratory congestion, or lower respiratory symptoms 2.
Multiple investigations confirm that milk consumption does not exacerbate asthma symptoms, and no relationship between milk consumption and asthma occurrence can be established 3.
The Perception vs. Reality Gap
The disconnect between belief and evidence is striking:
27.5% of study participants believed milk produces mucus, yet those who held this belief did not actually produce higher levels of nasal secretions when objectively measured 2.
Believers in the milk-mucus theory report more respiratory symptoms after drinking milk, but this is a subjective perception not matched by objective measurements 3, 2.
The sensations reported by believers primarily involve throat coating and perceived thickness of saliva rather than actual mucus production—effects likely related to the texture and fat content of milk creating a temporary coating sensation 4.
Clinical Implications for Respiratory Disease
For patients with chronic respiratory conditions, unnecessary dietary restrictions can be harmful:
Health Canada recommends 2 servings of milk daily (0.5 L) for children 2-8 years and 3-4 servings (0.75-1 L) for children 9-13 years for healthy development, and these recommendations should not be altered based on asthma status 5.
Milk elimination is often attempted in asthmatic children despite lack of evidence, potentially compromising calcium intake and growth 5.
The only legitimate reason to restrict milk in respiratory patients is documented cow's milk allergy, which can rarely present with asthma-like symptoms—this is distinct from the mucus production myth 3.
Important Caveats
True cow's milk allergy (IgE-mediated) can cause respiratory symptoms including bronchospasm, but this is an allergic reaction, not mucus production, and affects only a small subset of individuals 3.
The sensory experience of milk coating the throat is real but temporary, lasting only minutes and involving salivary secretions rather than respiratory mucus 4.
Patients with aspiration risk or swallowing difficulties may require thickened feeds for safety reasons unrelated to mucus production 6.
Bottom Line for Clinical Practice
Advise patients and families that restricting milk to reduce mucus or respiratory symptoms has no scientific basis and may cause nutritional harm, particularly in growing children 5, 1. The widespread belief persists due to the temporary sensory coating effect of milk in the mouth and throat, which is misinterpreted as increased mucus production 4. Unless true milk allergy is documented, dairy products should remain part of a balanced diet even in patients with asthma, COPD, or other chronic respiratory conditions 3, 5, 1.