From what I read it says online the use by date is 5 years. It is composed of potassium chloride, fumeric acid, tricalcium phosphate and monocalcium phosphate.
I don't understand how these chemicals expire. Bacterial growth?, chemical reaction? If there is no chemical reaction how can these four chemicals become less potent or effective? Even if they absorb moisture so what, same thing happens when you swallow it.
Yet another mystery.