Florfenicol, a synthetic drug with chemical structure and spectrum of antibacterial activity similar to chloramphenicol, has been shown to be effective against a number of bacterial pathogens. However, there are increasing signs of florfenicol-resistant bacteria due to the misuse and overuse of florfenicol in aquaculture. In the present study, florfenicol had a higher bactericidal efficacy in the presence of myo-inositol, which may be due to the ability of myo-inositol to increase susceptibility of Aeromonas hydrophila to florfenicol. Furthermore, in two different infected models, co-administration of myo-inositol and florfenicol significantly reduced the bacterial load in the liver, kidney and spleen tissues of A. hydrophila-infected Cyprinus carpio, and greatly increased the survival rate of infected fish. Finally, it was also found that myo-inositol exhibited synergistic action with other antibiotic drugs including neomycin sulfate, ceftriaxone and enrofloxacin. The results obtained in this study suggest that myo-inositol as an efficient adjuvant to antibiotic drugs could be useful in increasing the antimicrobial activity of antibiotic drugs against A. hydrophila infection, and could also be useful to help decrease the occurrence of antibiotic overuse in aquaculture.