Dutasteride is chemically similar Finasteride. It inhibits the conversion of testosterone into DHT by blocking the enzyme, 5-alpha reductase. There are 2 forms of 5-alpha reductase, type 1 and type 2. Dutasteride blocks both of these isoenzymes, where Finasteride blocks mainly the type 2 isoenzyme.
With more effectiveness comes more potential for side effects.