depends how bad the leak is, its only supposed to be a temporary measure, for very small leaks and weeps, if the leak is real bad, all the rad weld will leak out with it too, so i'd really look into gettiong it fixed properly

if it is used incorrectly or stays in the system for too long it can cause water pump failure, and blockages in smaller waterways of the engine, headgasket, turbo etc.