A persistent dry cough can be caused by a number of different problems. Sometimes the cause isn't too worrisome -- for example, in a post-infectious cough -- but sometimes the cause is quite serious, as in cases of heart failure or cancer. In general, a persistent dry cough can originate from your lungs, your heart, your sinuses or your gastrointestinal system. It can also result from medication side effects. While all persistent coughs ought to be evaluated by a medical professional, there are a few warning signs that should prompt earlier attention.
Many dry coughs result from respiratory conditions of one kind or another. For example, if your cough was preceded by symptoms of a viral upper respiratory tract infection, or URTI -- nasal congestion, sore throat, generalized aches and pains -- it may be a "post-viral" cough. Alternatively, if it is persistent and occurring in fits, particularly if accompanied by vomiting or a post-cough "whoop" made as you inhale quickly and deeply, this may represent whooping cough.
If you have asthma, your dry cough may represent an asthma flare, even if it isn't accompanied by shortness of breath or wheezing. Finally, sometimes more serious diseases are to blame. These include chronic obstructive pulmonary disease, or COPD, interstitial lung disease, or ILD, and cancer. Of these diseases, COPD and cancer are principally caused by a long history of exposure to cigarette smoke, while ILD can occur spontaneously, or result from exposure to a variety of toxic materials -- for example, mold, or some medications, such as amiodarone (Cordarone).
If your heart isn't pumping properly, a condition called congestive heart failure, or CHF, can develop. CHF results in blood backing up into the lungs. The fluid component of blood then seeps out of your circulation and into spaces in the lung which are normally fluid-free, which causes coughing. CHF usually results from years of high blood pressure or coronary artery disease. Interestingly, a class of medications called angiotensin-converting enzyme, or ACE, inhibitors, which are often given to patients with CHF to prevent further cardiac deterioration, can cause a dry cough. So, if you already have CHF and are on an ACE-inhibitor, it can sometimes be difficult to decide whether your cough is due to your CHF, or due to your medication.
Chronic sinusitis, whether as a result of infection or allergy, can lead to post-nasal drip. The fluid in your sinuses drains into your nose, and from there, into the back of your throat. Once in the back of your throat, this mucous triggers a coughing reflex. Chronic sinusitis can be a difficult condition to diagnose, but is often identified by symptoms such as chronic headache and nasal congestion.
Gastroesophageal reflux disease, or GERD, is a condition in which stomach acid refluxes into the esophagus. If this acid travels high enough, it can wind up in your throat, a condition called laryngopharyngeal reflux, or LPR. This can irritate your throat and cause a cough in the same way as post-nasal drip. GERD is usually diagnosed on the basis of typical symptoms; the sensation of reflux, an acid taste in your mouth, or discomfort in your upper stomach all can suggest this disease. LPR may be suspected if you have a hoarse voice, dry cough or a constant need to clear the throat.
A dry cough lasting more than a week or so probably deserves a workup by a trained medical professional, particularly if it's getting worse or if you have no clear explanation for why it's occurring. That said, there are a few cases in which help should be sought sooner rather than later. These include a dry cough associated with weight loss, fatigue, shortness of breath, abnormal leg swelling or coughing up blood.