The most frequent adverse reactions to potassium iodide are stomach upset, diarrhea, nausea, vomiting, stomach pain, skin rash, and salivary gland swelling or tenderness. Less frequent adverse reactions include gastrointestinal bleeding, confusion, irregular heartbeat, numbness, tingling, pain or weakness in hands or feet, unusual tiredness, weakness or heaviness of legs, fever, and swelling of neck or throat. Thyroid adenoma, goiter, and myxedema are possible side effects.
Iodism or chronic iodine poisoning may occur during prolonged treatment or with the use of high doses. The symptoms of iodism include burning of mouth or throat, severe headache, metallic taste, soreness of teeth and gums, symptoms of head cold, irritation of the eyes with swelling of the eyelids, unusual increase in salivation, acneform skin lesions in the seborrheic areas, and rarely, severe skin eruptions. If symptoms of iodism appear, the drug should be withdrawn and the patient given appropriate supportive therapy.
Hypersensitivity to iodides may occur and may be manifested by angioedema, cutaneous and mucosal hemorrhage, and signs and symptoms resembling serum sickness, such as fever, arthralgia, lymph node enlargement, and eosinophilia.