Effects of Long Term Antibiotic Therapy on Exacerbation Rate in Stable COPD Patients

Inclusion Criteria: * Informed consent given * Confirmed COPD diagnosis * Severity of disease: Patients with a measured FEV1\<80% of predicted normal values. * At least one treated exacerbation (Patient recalls an episode of symptomatic worsening which was treated and was consistent with a COPD exacerbation) in the previous year. * Age: ≥ 45 years of age at screening. * Able to complete questionnaires for health status and symptoms and considered able to comply with the dosing regimen. * Patients willing to report exacerbations and attend for study visits. Exclusion Criteria: * Patients with a known diagnosis of active TB or other chronic respiratory disease in the judgement of the study doctor. * Hepatic or renal impairment as defined as LFTs \> 5XULN, and eGFR\<30 ml/min/1.73m2. * Patients with known hypersensitivity to Tetracyclines, the IMP and/or Placebo including their excipients. * Patients taking ongoing antibiotic therapy for COPD or other conditions. * Patients with uncontrolled clinically significant hypertension * Female patients who are pregnant or planning on becoming pregnant during the study, or are breastfeeding. * Patients with uncontrolled clinically relevant bradycardia, cardiac arrhythmias or cardiac insufficiency. * Clinically relevant abnormal electrolyes (sodium or potassium), renal function (urea and creatinine) or liver function (ALT, AST, ALP) that could interfere with the objectives of the trial or safety of the volunteer. * Patient taking clinically significant contraindicated medication, as per the SmPC for Doxycycline. * Use of another experimental investigational medicinal product within 3 months of study enrolment. If the IMP used was as part of the NIHR WP2 study then entry to WP3 after a 6 week washout period is permissible. * Patients with any other condition precluding enrolment in the trial, according to the assessment of the study doctor. This will be documented at screening