www.nhs.uk/conditions/cough/pages/introduction.aspx
When to see your GP
There's usually no need to see your GP if you or your child have a mild cough for a week or two
However, you should seek medical advice if you've had a cough for more than three weeks
your cough is particularly severe or is getting worse
you cough up blood or experience shortness of breath, breathing difficulties or chest pain
you have any other worrying symptoms, such as unexplained weight loss, a persistent change in your voice, or lumps or swellings in your neck
A persistent cough may be caused by:
a long-term respiratory tract infection, such as chronic bronchitis
asthma – this also usually causes other symptoms, such as wheezing, chest tightness and shortness of breath
an allergy
smoking – a smoker's cough can also be a symptom of COPD
bronchiectasis – where the airways of the lungs become abnormally widened
postnasal drip – mucus dripping down the throat from the back of the nose, caused by a condition such as rhinitis or sinusitis
gastro-oesophageal reflux disease (GORD) – where the throat becomes irritated by leaking stomach acid
a prescribed medicine, such as an angiotensin-converting enzyme inhibitor (ACE inhibitor), which is used to treat high blood pressure and cardiovascular disease
Rarely, a persistent cough can be a symptom of a more serious condition, such as lung cancer, heart failure, a pulmonary embolism (blood clot on the lung) or tuberculosis.