Solid oral composition for the treatment or prophylaxis of potassium and magnesium deficiency in skeletal and cardiac muscle containing about 3 to about 50 milliequivalent bioavailable potassium, about 0.1 to about 25 milliequivalent bioavailable magnesium, and a milliequivalent ratio of potassium to magnesium between about 2:1 and about 14:1, where the potassium is in a controlled release form, and methods of treating or preventing potassium and magnesium deficiency by oral administration of such compositions.