Gastrointestinal  symptoms are the most common side effects of potassium supplements, including nausea, vomiting, abdominal discomfort, and diarrhea.  Taking potassium with meals or taking a  microencapsulated form of potassium may reduce gastrointestinal side effects.
Hyperkalemia is the most serious adverse reaction to potassium.  Hyperkalemia occurs when potassium builds up faster than the kidneys can remove it. It is most common in individuals with renal failure.  Symptoms of hyperkalemia may include tingling of the hands and feet, muscular weakness, and temporary paralysis. The most serious complication of hyperkalemia is the development of an abnormal heart rhythm (arrhythmia), which can lead to cardiac arrest.
Although hyperkalemia is rare in healthy individuals, oral doses greater than 18 grams taken at one time in individuals not accustomed to high intakes can lead to hyperkalemia.