CONTEXT Chronic obstructive pulmonary disease (COPD) is a lung disease that is usually caused by smoking. People with COPD feel short of breath and have a chronic cough. The condition worsens steadily, limiting exercise and normal activities. These symptoms are caused by damage to the breathing tubes which carry air into the lungs and to the air sacs where oxygen crosses from the air into the blood. In the UK, COPD affects around 3.7 million people and causes over 30,000 deaths per year. It is the fourth most common cause of death worldwide. From time to time, COPD patients experience attacks, where they feel more breathless or cough up more phlegm (sputum). These attacks are called exacerbations and are often set off by infection. Exacerbations are major events for people with COPD. During an exacerbation, COPD patients need more treatment and may require admission to hospital. It can take weeks to recover fully from an exacerbation and some people never get back to how they were before the exacerbation. Unfortunately, despite current best treatment, many people die during the weeks, months and years after an exacerbation. New treatments are required to prevent deaths and speed up recovery after exacerbations. AIM The overall aim of our research is to see whether a drug called metformin can help COPD patients recover more quickly or survive longer after an exacerbation. Metformin is usually used to lower blood sugar in patients with diabetes. We have found that most patients admitted to hospital for COPD exacerbations have high blood sugar. The higher their blood sugar, the less well they recover from their exacerbation. We therefore think that using metformin to lower blood sugar in COPD patients during exacerbations could improve recovery. Metformin can also dampen down inflammation and mop-up harmful substances called free radicals. These effects could be beneficial for COPD patients with exacerbations. To test our theory we looked back at hospital records of COPD patients. We found that patients leaving hospital on metformin survived longer than those without metformin. This finding is encouraging, but must be interpreted cautiously, as there are several possible explanations. We now need to test the benefits of metformin in clinical trials. THE STUDY Patients admitted to hospital with COPD exacerbations will be invited to take part in the trial. Those who agree will receive all the usual treatments for COPD exacerbations. In addition, they will take a study treatment as a capsule twice daily for 1 month. Participants will be allocated by chance (randomisation) to receive metformin or no medicine (placebo) in the capsule. Neither we nor the participants will know who is taking what. This will allow us to make an unbiased assessment of the effects of metformin. We will assess the benefits of metformin by comparing blood glucose between patients during hospital admission and over the whole study using a test called fructosamine. We will check that people taking metformin don't become more unwell or develop dangerous side effects and that they can tolerate the tablets. We will also get some idea of whether metformin helps them recover more quickly from their COPD exacerbations. However this will need to be tested more thoroughly in the future in a larger group of patients. POTENTIAL BENEFITS At the end of this trial we will know whether metformin can lower blood sugar safely in patients with COPD exacerbations. The next step will be to test metformin in a larger number of COPD patients to see if it can help them to recover more quickly from their exacerbations. Metformin is cheap and widely available. If it works it could quickly be adopted as a new treatment for these patients.