Loss of national monetary policy

One currency can only be provided by one institution printing and setting the interest rate on money deposited or lent to banks. However the central bank is a lender of last resort, and is not, contrary to popular belief, involved in most transactions involving money. The interest rate set by the central bank only indirectly determines the interest rate customers has to pay on their bankloans. Given that private firms or persons see different risks with different lendings the interest rate can be very different from one investment to the next, from one person to the next, or one country to the next. In general lending to poor involves more risk, and you need higher interest to compensate for that risk. Still only one set of politics for interest can be made at any time by one central bank. In general a bigger currency area will in general have less chance of using interest rate to stabilize the economy, because different areas may be in different stages in the boom-bust cycle, and the interest rate setting must be a compromise between the interests of the different part of the currency area.