Laser efficiency is an important issue for mid-IR Sb-based semiconductor lasers. It has been the key limiting factor in the efforts to develop high power lasers in recent years. This paper reviews the basic aspects of the problem and discusses some recent results. A number of factors affect the efficiency, one of which common to many materials was a high internal loss that increases rapidly versus temperature. A major contribution to this internal loss is the large intervalence band carrier absorption that occurs in all mid- to-low-gap III-V semiconductors. Recent studies of both types of Sb-based laser materials with InAs-like valence band (InAsSb) and GaSb-like valence band (InAs/GaInSb/AlSb type-II quantum wells) have showed such a strong absorption. Coupled with the thermal effects, the internal loss behavior results in an efficiency roll-off that limits the power performance. Issues for the improvement of the efficiency are discussed.