What causes phlegm?

What causes phlegm?

An excessive amount of phlegm can feel uncomfortable and unpleasant – especially if it’s thick and builds up in your throat. But the truth is, your body requires a certain amount of mucus in order to stay healthy. If you’re producing more phlegm than usual lately, you may be wondering why. In this article, we take a closer look at what causes phlegm.

Where does phlegm come from?

Phlegm is a thick, gloopy type of mucus, and you may find that you experience more of it when you’re unwell, such as if you have a respiratory infection like the common cold[1]. A surge in phlegm production is your body’s immune response to illness as it works to trap and get rid of unwanted germs and bacteria. Your body may also produce more phlegm as a reaction to exposure to a particular allergen or pollutant[2].

Why do I have phlegm in my throat?

Phlegm often builds up in your throat, but why? In a nutshell, your body works extra hard to keep this type of mucus away from your lungs. Your lungs feature small, hair-like structures called cilia which keep this mucus moving, forcing the phlegm upwards into your throat. The build-up of phlegm can make you feel like you need to clear your throat constantly, and it’s likely that you’ll also have the urge to cough as a way to remove this mucus.

Phlegm is produced by the body for a variety of different reasons. Here are some of the most common:

Acid reflux

Acid reflux, which is commonly known as heartburn, refers to stomach acid travelling up the oesophagus towards the throat. This happens when the sphincter muscle at the end of the oesophagus relaxes when it shouldn’t. One of the symptoms of acid reflux is excessive phlegm at the back of the throat. Other symptoms include a cough, recurring hiccups, bad breath, nausea, bloating and hoarseness

Acid reflux can be caused by a number of different things, including:

  • Smoking
  • Spicy, fatty or acidic food
  • Alcohol
  • Pregnancy
  • Being overweight
  • Stress
  • Anti-inflammatories, such as ibuprofen

You may be able to treat or reduce acid reflux by making simple lifestyle changes, such as quitting smoking, changing your diet, losing weight, eating little and often and finding ways to relax. If you cannot relieve your symptoms, you should seek advice from a medical professional[3].

Allergic rhinitis

Excessive phlegm production can also be caused by allergic rhinitis. This refers to inflammation of the inside of the nose due to the immune system overreacting to allergens such as pollen, dust or animal dander.

Allergic rhinitis typically presents as cold symptoms like a blocked or runny nose, itchiness and sneezing. Reducing your exposure to allergens can help to alleviate your symptoms. It can also help to rinse your nose with a salt water solution.

Antihistamines can relieve symptoms, and sometimes doctors will prescribe corticosteroid sprays if nothing else is working[2].


Asthma is a chronic condition that involves the airways narrowing due to inflammation. Asthma and excessive phlegm production often go hand in hand. If you suffer from asthma and you are coughing up a lot of phlegm, it may indicate that your airways are inflamed. Other symptoms can include wheezing, breathlessness and coughing.

You should use a reliever inhaler to help to reduce inflammation and open up your airways. However, if you need this inhaler more than three times per week, you should seek advice from your doctor. Using your daily preventer inhaler can help you to keep inflammation to a minimum[4].

Infections, such as the common cold

When you have a bad cold or other respiratory infection, phlegm production can go into overdrive. You may notice that it is thicker or darker in colour than usual. You may also experience a sore throat, runny nose, a cough, sneezing and tiredness. Drinking plenty of fluids, gargling with salt water and using saline nasal sprays can all help to reduce phlegm production.

A cold will usually get better by itself in around 10 days. To recover quickly, you should rest as much as possible and try to stay warm and hydrated. If you have a sore throat, you may wish to take lozenges to help numb the pain and fight infection. If your respiratory infection is more serious and doesn’t get better without treatment, you may need to consult your doctor[1].

Certain lung diseases

There are a range of lung diseases that can cause phlegm production to increase. They include:

  • chronic bronchitis
  • cystic fibrosis
  • pneumonia
  • chronic obstructive pulmonary disease (COPD).

Smoking and infections are the most common causes of lung diseases[5]. Genetics can also play a significant role. Symptoms can range from a persistent cough, to shortness of breath, to coughing up blood or mucus. You may feel like you cannot exercise like you used to, feel tired all the time or experience unintentional weight loss. If you think your phlegm may be caused by a lung disease, it’s important that you seek advice from your doctor.


[1] https://www.nhs.uk/conditions/common-cold/

[2] https://www.nhs.uk/conditions/allergies/

[3] https://www.nhs.uk/conditions/heartburn-and-acid-reflux/

[4] https://www.nhs.uk/conditions/asthma/

[5] https://www.nhs.uk/common-health-questions/lifestyle/what-are-the-health-risks-of-smoking/