When the engine starts to use coolant, something is wrong. Normally, a car should not consume coolant, as in most cases this is a closed system. It may be that after 30,000/40,000 kilometers a little bit is consumed, but often to nil.
Is there significantly less coolant in the reservoir? Then there is a good chance that there is a coolant leak. Immediately contact a garage, because driving with too little coolant can cause major damage. In some cases, the leak can also be solved yourself with a radiator stop leak additive, this depends on the size of the leak.