Loss of national monetary policy

With one currency, there can only be one interest rate. This results in rendering each present currency area unable to choose the interest rate which suits its economy best. If, for example, the European Union were to have an economic boom while the United States slumped into a depression, this period would be eased if each could choose the interest rate which best fitted its needs (in this case, a relatively high interest rate in the former, and a relatively low one in the latter).