We present the case of a 45-year-old man with atrial fibrillation and morbid obesity (weight 128 kg, height 168 cm, BMI 45.4) who was switched from Warfarin 5 mg once daily to Apixaban 5 mg twice daily because he did not achieve at least 60% of the time in therapeutic range. We performed serial determinations of apixaban plasma concentration (at 2, 6, 12, 24 hrs after intake) showing drug levels within reference range, even when the patient lose weight.