<p>Resistance of <i>Rhipicephalus microplus</i> to macrocyclic lactones represents a major challenge for the control of this ectoparasite, leading to substantial economic losses. The indiscriminate use of acaricides has resulted in increasingly resistant populations, highlighting the need for alternative control strategies and pharmacological approaches, such as the use of synergists to enhance acaricide efficacy against resistant ticks. This study evaluated the effect of piperonyl butoxide (PBO) as a synergist in modulating resistance to ivermectin, eprinomectin, and moxidectin in five tick populations from the semi-arid area of Northeast Brazil. Bioassays revealed widespread resistance, with variation in LC50 values among locations, likely reflecting differences in the intensity and frequency of macrocyclic lactone use and, consequently, differences in resistance intensity rather than evidence of distinct resistance mechanisms. The association of PBO with macrocyclic lactones resulted in significant synergism in the studied tick populations. A notable reduction in LC50 was observed in 3 out of 5 populations for ivermectin, 4 out of 5 populations for eprinomectin, and 3 out of 5 populations for moxidectin. Even in highly resistant populations to moxidectin, a marked decrease in LC50 was observed after PBO exposure. These results indicate a pharmacological interaction between PBO and macrocyclic lactones, consistent with a partial contribution of oxidative detoxification to the resistance phenotype. However, the absence of synergism in some populations and the persistence of high resistance levels suggest that additional mechanisms are also involved.</p>