Water . Water intoxication is uncommon, but is a potential cause for dangerous and even fatal arrhythmias. It is very difficult to consume such high quantities of water that it becomes dangerous. Massively excessive water intake results in dilution of the blood's critical electrolytes. This can result in instability of the heart's electrical system, resulting in arrhythmia. Such cases have been reported in marathoners and other athletes who excessively hydrate during competition.