The Central Bank of Nigeria spent $4 billion from the nation’s external reserves to defend the local currency in 12 months.

This is according to statistics contained in the CBN website.
 
The data also revealed that the external reserves would likely be closing the year on the $25bn mark.

The last day of the year of 2015, the external reserves recorded $29.69bn.
 
The defence of the naira by the CBN has come under severe criticism by economists, who believe that the forces of demand and supply should be allowed to determine the exchange rate of the naira.