Persistent cough is a common symptom, accounting for about 20% of referrals to outpatient chest clinics. Most coughs are caused by self-limiting viral infections such as the common cold. However, 1 in 4 people with a viral infection develop a persistent cough, which can go on for several weeks. Whooping cough is a common cause of persistent cough in young people and adults. Although the whooping cough vaccine gives lifelong protection against severe infection, it does not appear to give such long-term protection against milder infections, which can make someone cough for many weeks. There are currently no proven efficacious treatments for persistent cough following either a viral infection or infection with whooping cough.

Montelukast is a medication which is already licensed for the treatment of asthma. It works by blocking the action of chemicals called leukotrienes, which make the airways of people with asthma inflamed and sensitive. There is strong evidence to suggest that leukotrienes are also involved in causing persistent cough following viral or whooping cough infection. Montelukast may therefore also help settle persistent coughs in these settings.

Over 18 months, we will recruit patients aged 16-49 years with a cough lasting 2-8 weeks from general practices in England. An oral fluid sample will be taken from each participant to be tested for whooping cough. Participants will be randomly allocated to receive a 28-day course of montelukast or placebo tablets and asked to complete a daily cough diary for two weeks. They will be assessed after two weeks by their GP (face-to-face) and after four weeks by another member of practice clinical staff (telephone). Some participants will be given a 24-hour cough monitor to wear on study entry and at two-week follow-up. This study will be funded by the National Institute for Health Research's School of Primary Care.

Inclusion Criteria:

• Male or Female, aged 16 years to 49 years inclusive.
• Presenting with a persistent cough of 2-8 weeks' duration without an established diagnosis (e.g. asthma, gastro-oesophageal reflux).
• Able to complete cough diary and study questionnaires.

Exclusion Criteria:

• There is a contraindication to montelukast.
• Chronic severe disease which may cause persistent cough (eg cystic fibrosis, bronchiectasis, cardiac failure).
• Immunodeficiency/immunocompromised state.
• Pregnancy.
• Breastfeeding.
• Current smoker (i.e. stopped smoking less than 6 months ago).
• Regular medication associated with persistent cough (ACE inhibitors).
• The individual is in another clinical research study.