A cough that has outlasted three weeks is usually not a lingering infection, and this is the reason it so often fails to respond to repeated courses of antibiotics.
Most persistent coughs have an identifiable cause. Once that cause is found, the cough generally settles — sometimes within a week or two. The difficulty is that the cause is frequently not in the lungs at all.
How doctors classify cough duration
In clinical practice, cough is divided by duration:
- Acute cough — under three weeks, usually viral, usually self-resolving
- Subacute cough — three to eight weeks, often the tail end of an infection
- Chronic cough — beyond eight weeks, which warrants proper evaluation
The three-week mark is when it stops being reasonable to simply wait and hope it clears on its own.
First: Tuberculosis has to be excluded
In India this comes before everything else on the list.
A cough lasting two weeks or more is the national threshold for presumptive TB. Anyone with a cough of that duration should have TB considered and, where indicated, tested for — with a chest X-ray and a sputum test such as GeneXpert, which returns a result within a day or two.
This matters particularly in districts with higher TB burden. TB does not only affect people who look unwell or malnourished. It affects working adults with normal weight and no other symptoms. It is entirely treatable, and it is missed most often when nobody thinks to test.
If your cough has lasted more than two weeks and TB has not been considered, raise it yourself with your doctor.
The three most common causes of chronic cough
Upper airway cough syndrome (post-nasal drip)
Once TB and other serious causes are excluded, the majority of persistent coughs come down to three things, alone or in combination:
- Mucus from the nose and sinuses drips into the back of the throat and irritates it constantly.
- Clues: a sensation of something trickling down the throat, frequent throat clearing, worse in the morning or when lying down, a history of allergies or sinus trouble.
Asthma, including cough-variant asthma
Some people with asthma never wheeze and never feel breathless. Their only symptom is a cough. This is cough-variant asthma, and it is regularly missed because it does not look like classic asthma.
Clues: cough worse at night or in the early hours, triggered by cold air, dust, exercise, smoke or strong smells, often seasonal, sometimes a personal or family history of allergy or eczema.
Acid reflux (GERD)
Stomach acid rising into the oesophagus can trigger a persistent cough. Many people with reflux-related cough have no heartburn at all, which is why it gets overlooked.
Clues: cough worse after meals or when lying flat, hoarseness, a sour taste, throat clearing, sometimes worse after a heavy or late dinner.
Other causes worth knowing
A comprehensive evaluation also looks for these important causes:
- Blood pressure medication: ACE inhibitors — drug names ending in -pril, such as ramipril or enalapril — cause a persistent dry cough in a meaningful proportion of people who take them. It can begin months or even a year after starting. It resolves when the drug is changed, but only if someone makes the connection. Do not stop it yourself — ask your doctor about switching.
- Post-infectious cough: Some coughs genuinely do persist for six to eight weeks after a viral illness, as the airways stay hypersensitive. This settles on its own, but it is a diagnosis of exclusion, not an assumption.
- Smoking, and biomass smoke: Chulha and firewood smoke exposure over years causes the same airway damage as cigarettes and is a major cause of chronic cough and COPD in rural households, particularly among women.
- COPD: Especially with a morning cough productive of phlegm.
- Bronchiectasis: Permanently widened airways, often following childhood infection or TB, producing large volumes of sputum and recurrent chest infections.
- Interstitial lung disease: Usually a dry cough with progressive breathlessness.
- Lung cancer: A less common cause of chronic cough, but the reason a changing cough in a smoker is always investigated.
Warning signs that need prompt assessment
Do not wait with any of the following red-flag symptoms:
- Coughing up blood (haemoptysis), in any quantity
- Unexplained weight loss
- Fever or night sweats persisting
- Increasing breathlessness
- Persistent hoarseness beyond three weeks
- Chest pain with the cough
- A change in the character of a long-standing smoker's cough
- Difficulty swallowing
These do not mean something serious is present. They mean the question should be answered quickly rather than watched.
Why antibiotics usually don't work
Most persistent coughs are not bacterial. Post-nasal drip, asthma and reflux do not respond to antibiotics, and neither does a post-viral cough.
Repeated antibiotic courses for a cough are common, understandable, and largely ineffective. They also drive antibiotic resistance, which in a country with India's TB and pneumonia burden carries consequences well beyond the individual patient. If a third course has been prescribed without a diagnosis, the missing step is investigation, not another prescription.
What proper assessment involves
A thorough respiratory assessment covers:
- A detailed history — timing, triggers, sputum, associated symptoms, medications, exposures, occupation
- Chest X-ray, and where indicated a CT
- Sputum testing for TB, including GeneXpert where appropriate
- Spirometry with reversibility testing to identify asthma or COPD
- FeNO testing, which measures airway inflammation and can identify the steroid-responsive type of asthma
- Assessment for reflux and for nasal or sinus disease
- Review of blood pressure medication
Frequently more than one cause is present at once, which is why coughs sometimes improve only partially when a single problem is treated.
What you can do before your appointment
Help your specialist reach an accurate diagnosis sooner:
- Note when the cough is worst — night, morning, after meals, at work, outdoors
- List every medication, including blood pressure drugs
- Note exposures — smoking, cooking smoke, dust at work, birds at home, damp
- Bring previous X-rays and reports, and any old CT scans on CD
- Bring your inhalers if you use any, so technique can be checked directly
- Note whether anyone at home has a chronic cough or diagnosed TB
When to see a pulmonologist
Consider specialist assessment if your cough has lasted more than three to four weeks and has not been explained, if antibiotics have not helped, if it disrupts your sleep, if you have any of the warning signs above, or if you have been treated for asthma or reflux without improvement.
A cough is a symptom, not a diagnosis. The goal of the consultation is to find out what is causing it — and in most cases that is entirely achievable.