Does yellow mucus mean you need antibiotics? What phlegm can – and can’t – say about your health

When you’re sick you’ll often produce more phlegm, and might notice it’s thicker or a different colour: white, green, yellow or maybe even brown.

What can this phlegm – also called mucus, snot, sputum, catarrh and booger – really tell us about our health?

Here’s what to look for, and when to see a doctor.

What is phlegm?

We all produce mucus, even when healthy. Mucus is a barrier to the outside world that helps protect the organs in our bodies.

It’s produced by special cells within the epithelium. This is a tissue that lines the organs, cavities and surfaces in your body, including your eyes, mouth, digestive system and respiratory system (nasal passages and lungs).

Cells in this epithelial lining constantly secrete mucus. It acts as:

• a lubricant, preventing tissues from drying out

• a physical barrier filtering out and trapping particles such as dust, allergens and bacteria

• an immune barrier which contains various antimicrobial molecules that can kill a wide range of bacteria, fungi and even viruses.

Why do we produce more mucus when we’re sick?

The epithelium is one of the body’s first lines of defence when we have an infection. When these cells detect a pathogen – whether bacteria or a virus – they produce more mucus to strengthen the body’s physical and immune barriers.

Your body’s immune response causes inflammation, and this makes you produce more mucus. This excessive production and build-up is called mucus hypersecretion.

We often feel this is in our nasal passages when we have a cold, and sometimes lungs if we have a chest infection.

Usually, the epithelium’s cilia – long, hair-like cells – propel this mucus away and keep our airway passages clear.

But infections might damage or over-work the cilia, making us need to cough up phlegm or blow our noses to deal with this build-up.

Allergies are similar – your immune system overreacts to harmless substances (such as dust, pollen and certain foods) and produces excessive amounts of mucus.

Hypersecretion also affects people with chronic illness such as asthma, cystic fibrosis and chronic obstructive pulmonary disease.

Why does mucus get thicker?

Being sick can also affect the consistency of phlegm.

In a healthy person, mucus is around 90–95% water – the rest is made up of proteins called mucins and salts. This is why snot is usually clear and runny.

But when you’re sick, or recovering from an infection, studies suggest you produce more mucin proteins. These make your phlegm thicker.

This can give you a stuffy head, blocked nose or mean mucus accumulates and trickles to the back of your throat. This is known as post-nasal drip – also a common source of cough.

What about colour?

Thick mucus that is white or cloudy is usually an early sign of an infection, such as a cold. It may also indicate inflammation from allergies or chronic conditions such as asthma.

In the later stages of infection, snot is generally green, yellow or even brown. The greenish-yellow tinge comes from an enzyme called myeloperoxidase produced by immune cells that are fighting the infection.

Brownish-red mucus usually means it contains blood. This may happen when your nasal passages are damaged or irritated, often from blowing your nose a lot or because they’re dry.

This usually isn’t a cause for concern when blowing your nose. But if you cough up blood (haemoptysis) it can indicate something more serious, including a serious lung infection or even cancer. You should speak to a medical professional.

Black mucus is rare. It can be the sign of a fungal infection, or exposure to pollutants such as coal, dust or cigarette smoke. You should see a doctor if your mucus is black.

So, do I need antibiotics?

Your doctor may ask about your phlegm to make a diagnosis – its colour, consistency and how much you’re producing.

But studies show patients’ descriptions might not always be consistent or match observations made by a health-care professional.

So, a doctor may use a sputum colour chart to differentiate between viral and bacterial infections and decide whether to prescribe antibiotics.

Some evidence suggests yellowish-green mucus might be caused by a bacterial infection, rather than a virus (which won’t respond to antibiotics).

However, it’s not always the case.

So, the colour of your phlegm is not enough by itself to accurately diagnose an infection and tell if you need antibiotics.

But if you’re producing a lot of mucus and the colour or consistency is different from normal, it could be the sign your body is fighting an infection.

Drinking lots of fluids or use of decongestant sprays may help in the short term.

However, if you have additional symptoms, such as a fever, fatigue or loss of appetite for more than one to two weeks, speak to a health-care professional.