It is widely recognized that warfarin undergoes predominant metabolism through CYP2C9, whereas tolvaptan is metabolized by CYP3A4.8 Reportedly, the blood concentration of tolvaptan decreased to only 13% during rifampicin treatment.9 In patients with HF, decreased drug absorption due to gastrointestinal oedema may affect bioavailability. The gene discussed is CYP2C9; the disease is hydrops fetalis.