A persistent cough can be embarrassing, especially if people think you have COVID.

Coughing frequently can also make you physically tired, interfere with sleep^[1] and trigger urinary incontinence^[2]. As a GP, I have even treated patients whose repetitive forceful coughing has caused stress fractures in their ribs.

So, why do some coughs linger so long? Here are some of the most common causes – and signs you should get checked for something more serious.

Why do we cough?

The cough reflex is an important protective mechanism. Forcefully expelling air helps clear our lungs and keep them safe from irritants, infections and the risk of choking.

Some people who have long-term conditions, such as chronic bronchitis or bronchiectasis, have to cough frequently^[3]. This is because the lung’s cilia – tiny hair-like structures that move mucus, debris and germs – no longer work to clear the lungs.

A wet or “productive” cough means coughing up^[4] a lot of mucus.

A cough can also be dry or “unproductive^[5]”. This happens when the cough receptors in the airways, throat and upper oesophagus have become overly sensitised, triggering a cough even when there’s no mucus to clear.

Causes of a chronic cough

A cough is considered chronic^[6] when it lasts longer than eight weeks in adults, or four weeks in children.

The three most common causes^[7] are:

• post-nasal drip^[8] (where mucus drips from the back of the nose into the throat)
• asthma
• acid reflux from the stomach.

These often go together. One study^[9] found 23% of people with chronic cough had two of these conditions, and 3% had all three.

This makes sense – people prone to airway allergies are more likely to develop^[10] both asthma and hayfever (allergic rhinitis). Hayfever is probably the main cause^[11] of persistent post-nasal drip.

Meanwhile, prolonged, vigorous coughing can also cause reflux^[12], possibly triggering further coughing.

Chronic cough is the primary symptom of two other conditions, although these can be more challenging to diagnose^[13]: cough-variant asthma and eosinophilic bronchitis. Both conditions inflame the airways^[14]. However, they don’t rapidly improve with ventolin (the standard clinic test to diagnose asthma).

Allergies can cause inflammation that triggers a chronic cough. Kmpzzz/Shutterstock^[15]

Coughs after respiratory infections

Coughs can also persist long after^[16] a viral or bacterial infection. In children with colds, one systematic review^[17] found it took 25 days for more than 90% to be free of their cough.

After an infection, cough hypersensitivity may develop^[18] thanks to inflamed airways and over-responsive cough receptors. Even minor irritants will then trigger the coughing reflex.

The body’s response to infection makes the mucus more sticky^[19] – and more difficult for the overworked, recovering cilia to clear. Allergens in the air can also more easily penetrate the upper airway’s damaged lining.

This can trigger an unhelpful feedback loop that slows the body’s recovery after an infection. Excessive and unhelpful coughing tends to further fatigue the recovering cilia and irritate the airway lining.

Could I still have an infection?

When a cough persists, a common concern is whether a secondary bacterial infection^[20] has followed the first viral infection, requiring antibiotics.

Simply coughing up yellow or green phlegm is not enough to tell.

To diagnose a serious chest infection, your doctor will consider the whole picture of your symptoms. For example, whether you also have shortness of breath, worsening fever or your lungs make abnormal sounds through a stethoscope.

The possibility you have undiagnosed asthma or allergies should also be considered.

Read more: Health Check: why do I have a cough and what can I do about it?^[21]

What treats a persistent cough?

People with a persistent cough who are otherwise healthy may request and be prescribed^[22] antibiotics. But these rarely shorten how long your cough lasts, as irritation – not infection – is the primary cause of cough.

The most effective treatments for shifting sticky mucus from the airways are simple ones: saline^[23] nose sprays and washes, steam inhalation and medicated^[24] sore throat sprays.

Honey has also been shown^[25] to reduce throat irritation and the need to cough.

The effectiveness of cough syrup is less clear^[26]. As these mixtures have potential side effects, they should be used with care.

The most effective treatments are simple ones, including steam inhalation. New Africa/Shutterstock^[27]

Signs of something more serious

Sometimes, a cough that won’t go away could be the sign of a serious condition, including lung cancer or unusual infections. Fortunately, these aren’t common.

To rule them out, Australia’s chronic cough guidelines^[28] recommend a chest x-ray and spirometry (which tests lung volume and flow) for anyone presenting to their doctor with a chronic cough.

You should seek prompt medical attention if, in addition^[29] to your cough^[30], you:

• cough up blood
• produce a lot of phlegm
• are very short of breath, especially when resting or at night
• have difficulty swallowing
• lose weight or have a fever
• have recurring pneumonia
• are a smoker older than 45, with a new or changed cough.

What if there’s no clear cause?

Very occasionally, despite thorough testing and treatment, a cough persists. This is called refractory chronic cough.

When no cause can be identified, it’s known as unexplained chronic cough. In the past, unexplained cough may have been diagnosed as a “psychogenic” or “habit” cough, a term which has fallen from favour^[31].

We now understand that cough hypersensitivity makes a person cough out of proportion to the trigger, and that both the peripheral and central nervous systems play a role^[32] in this. But our understanding of the relationship^[33] between hypersensitivity and chronic cough remains incomplete.

These are disabling conditions^[34] and should be referred to a respiratory clinic or a chronic cough specialist. Speech pathology treatments may also be effective for refractory and unexplained coughs^[35].

There are a class of new medications^[36] in the pipeline that block cough receptors, and seem promising for persisting, troublesome coughs.

References

1. ^{^} interfere with sleep (doi.org)
2. ^{^} trigger urinary incontinence (doi.org)
3. ^{^} have to cough frequently (doi.org)
4. ^{^} coughing up (jtd.amegroups.org)
5. ^{^} unproductive (mrmjournal.biomedcentral.com)
6. ^{^} cough is considered chronic (lungfoundation.com.au)
7. ^{^} three most common causes (www.uptodate.com)
8. ^{^} post-nasal drip (my.clevelandclinic.org)
9. ^{^} One study (www.atsjournals.org)
10. ^{^} to develop (www.nationalasthma.org.au)
11. ^{^} main cause (pubmed.ncbi.nlm.nih.gov)
12. ^{^} cause reflux (journal.chestnet.org)
13. ^{^} challenging to diagnose (pmc.ncbi.nlm.nih.gov)
14. ^{^} inflame the airways (doi.org)
15. ^{^} Kmpzzz/Shutterstock (www.shutterstock.com)
16. ^{^} can also persist long after (www1.racgp.org.au)
17. ^{^} systematic review (pubmed.ncbi.nlm.nih.gov)
18. ^{^} hypersensitivity may develop (doi.org)
19. ^{^} makes the mucus more sticky (doi.org)
20. ^{^} secondary bacterial infection (www.betterhealth.vic.gov.au)
21. ^{^} Health Check: why do I have a cough and what can I do about it? (theconversation.com)
22. ^{^} request and be prescribed (www1.racgp.org.au)
23. ^{^} saline (doi.org)
24. ^{^} medicated (www.japsr.in)
25. ^{^} has also been shown (doi.org)
26. ^{^} less clear (bmjopenrespres.bmj.com)
27. ^{^} New Africa/Shutterstock (www.shutterstock.com)
28. ^{^} chronic cough guidelines (lungfoundation.com.au)
29. ^{^} in addition (link.springer.com)
30. ^{^} your cough (pmc.ncbi.nlm.nih.gov)
31. ^{^} fallen from favour (journal.chestnet.org)
32. ^{^} play a role (www.nature.com)
33. ^{^} the relationship (www.nature.com)
34. ^{^} disabling conditions (doi.org)
35. ^{^} refractory and unexplained coughs (doi.org)
36. ^{^} new medications (doi.org)