The monitoring results in China have shown that the phenomenon of single-pollutant residues exceeding the standard in food has decreased, while the coexistence of many low-level residual pollutants has increased significantly. Among these, the combined use of enrofloxacin (EF) and tilmicosin (TIM) is serious. Despite that, little is comprehended about influences of the drug-drug interactions (DDIs) caused by EF and TIM. The purpose of this work is to evaluate the effects caused by the combination of these two drugs on metabolism and residues in vivo and in vitro. The results showed that TIM affected the metabolism of EF by inhibiting CYP3A4 and increased the residual concentrations of EF in broilers. Moreover, the time of elimination of EF was prolonged. Thus, the combined use of TIM and EF must be reduced in veterinary drugs and feeds in order to ensure the safety of humans and animals