Hypokalemia is a low level of potassium (K⁺) in the blood serum. Mild low potassium does not typically cause symptoms. Symptoms may include feeling tired, leg cramps, weakness, and constipation. Low potassium also increases the risk of an abnormal heart rhythm, which is often too slow and can cause cardiac arrest.

Causes of hypokalemia include vomiting, diarrhea, medications like furosemide and steroids, dialysis, diabetes insipidus, hyperaldosteronism, hypomagnesemia, and not enough intake in the diet. Normal potassium levels in humans are between 3.5 and 5.0 mmol/L (3.5 and 5.0 mEq/L) with levels below 3.5 mmol/L defined as hypokalemia. It is classified as severe when levels are less than 2.5 mmol/L. Low levels may also be suspected based on an electrocardiogram (ECG). The opposite state is called hyperkalemia, which means a high level of potassium in the blood serum.

The speed at which potassium should be replaced depends on whether or not there are symptoms or abnormalities on an electrocardiogram. Potassium levels that are only slightly below the normal range can be managed with changes in the diet. Lower levels of potassium require replacement with supplements either taken by mouth or given intravenously. If given intravenously, potassium is generally replaced at rates of less than 20 mmol/hour. Solutions containing high concentrations of potassium (>40 mmol/L) should generally be given using a central venous catheter. Magnesium replacement may also be required.

Hypokalemia is one of the most common water–electrolyte imbalances. It affects about 20% of people admitted to the hospital. The word hypokalemia comes from hypo- 'under' + kalium 'potassium' + -emia 'blood condition'.

Mild hypokalemia is often without symptoms. Acute potassium depletion through severe dietary restriction leads to hypokalemia and elevation of blood pressure. Severe hypokalemia, with serum potassium concentrations of 2.5–3 meq/L (Nl: 3.5–5.0 meq/L), may cause muscle weakness, myalgia, tremor, and muscle cramps (owing to disturbed function of skeletal muscle), and constipation (from disturbed function of smooth muscle). With more severe hypokalemia, flaccid paralysis and hyporeflexia may result. Reports exist of rhabdomyolysis occurring with profound hypokalemia with serum potassium levels less than 2 meq/L. Respiratory depression from severe impairment of skeletal muscle function is found in some people. Psychological symptoms associated with severe hypokalemia can include delirium, hallucinations, depression, or psychosis.

Hypokalemia can result from one or more of these medical conditions:

Not eating a diet with enough potassium-containing foods or fasting can cause the gradual onset of hypokalemia. This is a rare cause and may occur in those with anorexia nervosa or those on a ketogenic diet.^{[citation needed]} In a short-term study, severely low potassium intake (10 meq/day) led to mild hypokalemia at 9 days (mean serum potassium 3.2 meq/L).

A more common cause is excessive loss of potassium, often associated with significant fluid losses that cause potassium to be flushed out of the body. Typically, this is a consequence of diarrhea, excessive perspiration, losses associated with crush injury, or surgical procedures. Vomiting can also cause hypokalemia, although relatively little potassium is lost in the vomitus. Rather, heavy urinary losses of K⁺ in the setting of post-emetic bicarbonaturia force urinary potassium excretion. (See discussion of alkalosis below.) Other gastrointestinal causes include pancreatic fistulae and the presence of adenoma.^{[citation needed]}