A cough that won't quit is one of the most common reasons people end up searching the internet at 2am, half-convinced something is seriously wrong. In most cases it isn't, but "most cases" isn't the same as "every case", and knowing roughly how long a cough should reasonably last, and what tends to keep one going, makes it much easier to judge when to sit tight and when to get it looked at.
Acute, subacute and chronic cough: what the timeline means
Clinicians generally sort a cough into three timeframes, and the labels matter because they point toward different likely causes. An acute cough lasts under three weeks and is almost always down to a cold, flu or other short-lived chest infection. A subacute cough runs from three to eight weeks, often this is simply the tail end of a viral infection settling down more slowly than the rest of your symptoms, sometimes called a post-infectious cough. A chronic cough is one that persists beyond eight weeks, and at that point the cause is much less likely to be "just a lingering cold" and more likely to be something that genuinely needs identifying and treating in its own right.
These aren't arbitrary cut-offs. They roughly track how long it takes airway inflammation from a simple viral infection to settle on its own, versus how long an underlying condition like asthma or reflux would keep triggering a cough indefinitely if left unaddressed. Using the three-week mark as your own personal checkpoint is a reasonable, practical habit: if a cough is still going strong at that point, it's worth a conversation with a pharmacist or GP rather than waiting it out further.
Common causes of a cough that lingers after a cold
The single most common reason a cough overstays its welcome is straightforward post-infectious irritation. A cold or flu can leave the airways temporarily more sensitive than usual, so they react to cold air, smoke, dust or even talking with a bout of coughing, despite the original infection being long gone. This tends to gradually ease over a few weeks without any specific treatment, though it can feel disproportionately stubborn while it lasts.
A chesty, productive cough with green or yellow phlegm that develops partway through a cold can point to a secondary chest infection rather than the original virus. Most of these still resolve without antibiotics, but a fever that returns after initially improving, or phlegm that changes colour and volume, is worth mentioning to a pharmacist or GP, since it helps them judge whether antibiotic treatment is appropriate for your particular picture.
Smoking, and exposure to secondhand smoke, vaping, or workplace dust and fumes, all prolong and worsen a cough regardless of what started it, largely because they keep the airways irritated on top of whatever the underlying cause is. Cutting exposure where you can genuinely speeds up recovery.
Asthma, reflux and postnasal drip: the three most missed causes
When a cough moves from subacute into truly chronic territory in someone who doesn't smoke, three causes account for the large majority of cases, and a lot of people are surprised to hear any of them mentioned as a cough trigger at all.
Asthma doesn't always announce itself with wheeze and breathlessness: a persistent dry cough, particularly one that's worse at night, after exercise, or around cold air and allergens, can be the main or even only symptom in some people. This pattern is sometimes described as cough-variant asthma.
Postnasal drip, more formally called upper airway cough syndrome, happens when mucus from the nose and sinuses trickles down the back of the throat and triggers a repeated urge to clear it or cough, especially when lying down at night or first thing in the morning. It's often linked to allergies, hay fever, or a lingering sinus irritation, and can be easy to overlook because the nasal symptoms themselves may feel minor.
Acid reflux is the third major cause, and one of the least intuitive: stomach acid or its vapour can irritate the airway and trigger coughing without any of the classic burning heartburn sensation in the chest. This is sometimes called silent reflux, and it catches people off guard because there's no obvious digestive symptom pointing them toward the real cause.
Because these three causes can overlap, or occur without their "typical" accompanying symptoms, working out which one (or which combination) is behind a chronic cough often takes a bit of trial and assessment rather than a single test giving an instant answer.
Medicines and other causes worth ruling out
A cough that started not long after beginning a new medicine is worth flagging specifically. ACE inhibitors (a widely used class of blood pressure medicine including drugs such as ramipril and lisinopril) cause a persistent, dry, tickly cough in a meaningful minority of the people who take them. It isn't dangerous, but it's genuinely uncomfortable, and it's a well-recognised side effect rather than a coincidence. If the timing lines up, mention it to your GP or pharmacist rather than assuming you simply have to live with it. A medication review, and a switch to an alternative blood pressure medicine if appropriate, usually resolves it.
Where reflux is suspected as the cause of a chronic cough, a trial of an acid-suppressing medicine such as omeprazole is sometimes used, partly as treatment and partly to help confirm reflux as the underlying trigger if the cough improves. This isn't usually a quick fix, it can take several weeks of consistent use before any improvement in a reflux-related cough becomes noticeable, and your clinician will advise based on your individual circumstances.
Self-care that can help while you wait it out
For a short-term cough that's simply the tail end of a cold, the evidence for over-the-counter cough syrups actually shortening how long it lasts is fairly limited, even though they remain popular. Simple measures often help just as much: staying well hydrated, using honey (not suitable for babies under one), sucking a sugar-free lozenge, keeping the air you breathe humidified, and avoiding smoke or other irritants while the airways settle. A pharmacist can talk through which over-the-counter options are reasonable for you personally, taking into account any other medicines you're taking and whether your cough is dry or productive.
If postnasal drip or hay fever-type symptoms seem to be contributing, treating the nasal side directly, for example with a steroid nasal spray or a non-drowsy antihistamine, sometimes settles the cough as a knock-on effect, since the two are often driven by the same underlying irritation.
Propping yourself up a little more at night, avoiding large meals close to bedtime, and cutting back on caffeine and alcohol can also help if reflux is a suspected contributor, alongside any medicine your clinician recommends.
Red flags and when to see a doctor
Most persistent coughs turn out to have a manageable, non-urgent cause. But a handful of features should prompt you to get seen promptly rather than continuing to wait it out: coughing up blood, unexplained weight loss, a persistently hoarse voice, chest pain, or breathlessness that's new or getting worse. A new or changed cough lasting more than three weeks in a current or former smoker over 40 also warrants a timely GP assessment rather than a "wait and see" approach, since these features are the ones UK clinical guidance specifically flags for closer investigation.
Outside of those red flags, the general rule of thumb holds: a cough that's still present and not clearly improving by three weeks is worth discussing with a pharmacist or GP, and one that's gone on beyond eight weeks should be properly assessed if it hasn't been already. Getting the underlying cause identified, whether that's asthma, reflux, postnasal drip, a medicine side effect, or something else, is usually far more effective than continuing to treat the cough itself in isolation.