IPF is a progressive scarring lung condition causing coughing and breathlessness. IPF patients often have reflux disease meaning stomach acid may be breathed into the lungs, potentially damaging them. Medicines which stop stomach acid production, proton pump inhibitors (PPIs), can be used to reduce reflux symptoms including heartburn. Some researchers suggest PPIs also reduce IPF progression. This research aims to see if IPF progresses slower if treated with PPIs. Based on the results, we will be able to recommend whether or not IPF patients should take PPIs. This trial will involve 298 IPF patients from approximately 37 UK hospitals. At the beginning of the study, we will ask patients to start performing weekly breathing tests at home using equipment provided, and ask those with a cough to use a device to count the number of times they cough in 24hours. We will ask them to answer two questions rating their coughing and breathlessness, and complete questionnaires on their coughing, IPF, sleep habits and general condition. People will be given a PPI, called lansoprazole, or dummy tablets, twice per day for 12 months. They will be given a leaflet telling them what to do about reflux symptoms. At the end of the study, we will repeat these tests and analyse the results. We will record any side effects people may get. If people suffer side effects, they can reduce the dose. People taking medicines that interact with PPIs or have other serious medical conditions won't be able to participate. People receiving PPIs will only be able to participate if they can stop taking their medication without their heartburn returning.