Signs of low potassium include abnormal heart rhythms or palpitations, fatigue or muscle weakness, constipation, muscle damage, and tingling or numbness, according to MedlinePlus. A minor decrease in potassium levels may not cause symptoms.
Potassium is necessary for nerve and muscle-cell function, including cells in the heart. The normal range for potassium is 3.6 to 5.2 millimoles per liter, states Mayo Clinic. A potassium level below 2.5 millimoles per liter is low and may cause life-threatening side effects.
A large drop often slows the heart rate, warns MedlinePlus. The initial signs of a slowed heart rate due to low potassium include feeling faint or lightheaded. In cases of extremely low potassium, the heart can stop completely.