Why would an antibiotic be listed with unclear efficacy?

Efficacy refers to the ability of the antibiotic to produce its intended result, that is, to cure the infection by killing the microbes of interest. There are many reasons why an antibiotic's efficacy may be unclear. These can include whether there is variable activity if the drug is being used off-label or not FDA approved, if its use is based on limited data, or if the drug is not commonly used for that particular microbe and infection.