The consequence of over-issuance is inflation.

Inflation is generally defined as: under the credit currency system, the amount of currency in circulation exceeds the actual needs of the economy, causing currency depreciation and a comprehensive and sustained rise in price levels - in more popular language. That is to say: within a given period of time, the price level in a given economy generally continues to increase, resulting in a continuous decline in the purchasing power of money.

In Keynesian economics, the reason is that changes in aggregate supply and aggregate demand in the economy lead to changes in the price level. In monetarist economics, the reason is: when the currency circulation in the market increases, people's monetary income increases and purchasing power decreases, which affects the rise in prices and causes inflation. The theory is summarized in a very famous equation: MV=PT.