If you are ill and you vomit or have diarrhoea, your body loses some of its essential potassium supply. If you have long QT syndrome or Brugada syndrome, having a low level of potassium can make your condition worse. Therefore if you have vomiting or diarrhoea for more than one day, you should add a rehydration sachet to some water and drink it. You can buy rehydration sachets such as ‘dioralyte’ at any chemist. If your vomiting and diarrhoea continue or if vomiting prevents you from keeping down your rehydration drink, you should go to hospital straight away to be assessed and potentially receive fluids through a drip. If you have long QT syndrome or Brugada syndrome, it is a good idea to have some sachets of a rehydration preparation at home just in case.