Can't shake that cough?

Dry. Hacking. Loose. Deep. Phlegmy. Barking. Chesty. A cough can sound like any of these descriptors — often more than one at the same time — and these characteristics can sometimes point toward its cause.

Now add another: lingering. What does that tell us? Not as much as we might hope, a Harvard expert says.

A natural reflex that helps keep our airways clear, the average cough — typically due to a cold or another respiratory virus — lasts three weeks or less, says Dr. Jessica McCannon, a pulmonary critical care specialist with Harvard-affiliated Mass General Brigham. When a cough endures for eight weeks or more, it's considered chronic. Only rarely is this dangerous, but it certainly can be mystifying.

"When people come to a lung doctor for a chronic cough, they usually hope that at the end of the first visit they'll leave with an answer and some medication and will quickly feel better," Dr. McCannon says. "But of all the things we care for, it's one of the most difficult. And if we don't talk about that up front, I think people will have ongoing frustration and disappointment."

Many possible culprits

Indeed, be prepared for a detective hunt. Many chronic coughs have more than one contributor, so doctors don't just look to the airway or lungs to unearth the cause.

"Often we're thinking top to bottom — about the sinuses, swallow function, and esophagus and stomach in addition to the airways and lungs themselves," Dr. McCannon says.

More than a dozen conditions or situations can explain a lingering cough, including

• sinus problems, such as sinusitis
• bronchitis
• seasonal allergies
• gastroesophageal reflux disease (GERD)
• side effects from medications, such as ACE inhibitors to treat high blood pressure.

But only a handful of chronic cough triggers are serious, such as

• asthma
• chronic obstructive pulmonary disease (COPD)
• interstitial lung disease, a progressive condition involving coughing and breathlessness
• chronic lung infections
• bronchiectasis, chronic inflammation of the tubes leading to the lungs
• lung cancer.

Occasionally, chronic coughs arise from "very weird causes" such as impacted earwax, Dr. McCannon points out. And sometimes the culprit is all too clear: smoking — whether tobacco, marijuana, or vaping. "Basically, anything going into the lungs that shouldn't be there will potentially cause a cough."

Seeking a diagnosis

It's reasonable to try over-the-counter medications to quell your cough. The choice depends on the apparent cause of the cough, so you might take cough suppressants, antihistamines if you think it may be allergies, or antacids for acid reflux as a possibility. But if they don't make a dent, Dr. McCannon says, it's time to see a health care professional. "Tell your doctor what's worked and what hasn't. That can give us some clues," she says.

Certain red flags make a doctor's visit more pressing. Schedule an appointment (or a follow-up) if your lingering cough doesn't resolve after treatment with antibiotics or if you also experience breathlessness; wheezing; chest pain, pressure, or tightness; blood in your sputum; weight loss; or fatigue.

"A lingering cough with any of those things should lead you to seek care soon," Dr. McCannon says.

Getting to the bottom of things will likely involve a physical exam, x-rays, or lung function testing. Your doctor may order CT imaging as well, "although that's certainly not necessary at the threshold of three months unless there are other warning signs," she says.

To make your doctor's visit more fruitful, Dr. McCannon suggests one important strategy: track your cough.

"Note its patterns. What circumstances bring it on? Is it productive or dry? Is it worse after meals, or when you're lying down? Does it intrude on sleep?" she asks. "Paying attention to the circumstances in which you notice it can provide us with some really helpful clues."

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