The purpose of this study is to investigate whether a 3 month treatment course of low-dose Colchicine decreases the recurrence of Atrial fibrillation (AF) after electrocardioversion (ECV) in patients with AF.

Atrial fibrillation is the most common cardiac arrhythmia worldwide and is associated with an increased risk of heart failure, stroke and death. Over the next 40 years the investigators expect another increase in the prevalence of atrial fibrillation with a risk of 1:3 in people over 65 years to develop atrial fibrillation. Electroconversion can occur in patients with atrial fibrillation reestablish sinus rhythm acutely with a controlled electrical shock. Unfortunately it is known however, that there is a short-term recurrence of atrial fibrillation in about 60%. This underlines that our current treatment options are inadequate. There is increasing evidence that inflammation is integral to initiation and maintenance of atrial fibrillation. Therefore, the researchers see inflammation as a possible therapeutic target to reduce the recurrence rate of atrial fibrillation after electroconversion. To test this hypothesis and to help patients, the investigators want to conduct the COLECTRO-AF study.

Inclusion Criteria:

• Age >18 years
• ECG-documented AF prior to ECV
• Successful ECV with conversion of AF to sinus rhythm with persistent sinus rhythm ≥30 minutes after ECV
• Ability to give written informed consent

Exclusion Criteria:

• AF persistence after cardioversion or early AF recurrence within 30 minutes after ECV
• Any other rhythm than AF before cardioversion
• Pulmonary vein isolation within 3 months prior to ECV or pulmonary vein isolation planned within 3 months after ECV
• Known intolerance or hypersensitivity to Colchicine
• Any other absolute indication for Colchicine intake
• Intake of a strong inhibitor of CYP3A4 or P-Glycoprotein (clarithromycin, erythromycin, telithromycin, cyclosporine, ketoconazole or itraconazole)
• Serious gastrointestinal disease (severe gastritis or diarrhea)
• Clinically overt hepatic disease
• Severe renal disease (eGFR< 30ml/min/1.73m2)
• Clinically significant blood dyscrasia (e.g., myelodysplasia)
• Significant immunosuppression (e.g. due to transplantation or rheumatic disease)
• Pregnant or breastfeeding women, or women of child-bearing potential who do not use a highly effective form of birth control
• Life expectancy <1 year

A randomization list will be provided by the Clinical Trial Unit Basel to an unblinded person at the sponsor's site. The unblinded person will allocate the randomization numbers. Patients and research staff involved in patient recruitment, data management, outcome adjudication and data analyses will fully be blinded