The season of constant coughing and running noses will very shortly be upon of us. In the next few upcoming few months, you’ll no doubt hear the much-touted advice to avoid drinking milk as it can make your cold even more mucusy. Equally, parents often tend to avoid giving kids with asthma milk as they fear it increases production of gooey phlegm from the lungs, making it even harder to breathe.

However, all of this is a myth, according to Dr Ian Balfour-Lynn, a children’s respiratory consultant from Royal Brompton Hospital in London.

As explained in a recent paper published in the journal Archives of Diseases of Childhood, this idea can be traced back to a text from the 13th century written by Moses Maimonides, a Jewish philosopher and court physician in Egypt. The 800-year-old text argues that certain foods – namely black beans, peas, heavy meats, and old cheese – can increase the production of phlegm and worsen asthma symptoms. He mentions that milk, in particular, can cause a “stuffing in the head.” He also recommends a bowl of hot chicken soup to help you get over a cold (which, obviously, only a fool would argue against). 

The milk-mucus myth is an idea that's also popped in traditional Chinese texts and continues to persist in modern medical literature, yet there’s very little clinical evidence to back it up. Numerous trials have looked into the milk-common cold hypothesis and produced fairly unconvincing results. By most accounts, there’s no link between drinking milk and symptoms of respiratory tract congestion or an increase in phlegm production.

“While certainly the texture of milk can make some people feel their mucus and saliva is thicker and harder to swallow, there is no evidence (and indeed evidence to the contrary) that milk leads to excessive mucus secretion,” Dr Balfour-Lynn wrote in the study.

Nevertheless, a huge portion of the public believes it. One study from the US found that nearly 60 percent of the public believed drinking milk can increase mucus production, with up to half saying they avoid giving milk to their children if they are sick. It’s even a myth that persists within trained medical professionals. 

“George Orwell said that myths which are believed in tend to become true,” Dr Balfour-Lynn added. “Our department has repeatedly been told by parents that drinking milk increases mucus production from the lungs, and so they stop their child having milk.”

“Milk is an important source of calories, calcium, and vitamins for children," he added. "The milk–mucus myth needs to be rebutted firmly by healthcare workers.”