Matching on a non-risk factor in the design of case-control studies does not always result in an efficiency loss.
It has been shown that matching on a non-risk factor may result in an efficiency gain in some situations. However, the gain is small and unlikely to occur in practice. Therefore, the popular recommendation of only matching on variables known a priori to be strong risk factors is quite valid from a practical standpoint.
D004526:Efficiency; D004812:Epidemiologic Methods; D012107:Research Design; D012306:Risk