Praziquantel (PZQ) is an anthelminthic drug that has been tested worldwide to treat parasitosis in farmed fish. Information regarding the applicability and sustainability of medications in tambaqui Colossoma macropomum farming is highly needed. This study investigated the effects of PZQ at different concentrations and exposure times on the hematological parameters of tambaqui and the efficacy against monogeneans. The toxicity test with PZQ was conducted at three exposure times (0.5 h, 1 h and 24 h). In each of them, five PZQ concentrations (3.12, 6.25, 12.5, 25 and 50 mg L−1) were evaluated, with a control group, in triplicate, with six fish per unit. The same assays were conducted to evaluate the toxicity of ethanol, used as solvent. Based on the toxicity test and an in vitro test, the efficacy of PZQ against monogeneans in tambaqui was tested in four treatments (6.25, 12.5, 25 and 35 mg L−1), with long bath duration (24 h), with a control group, in triplicate, with six fish per unit. Fish were sampled and subjected to hematological and parasitological analyses. The use of ethanol as solvent did not interfere significantly with the physiology of tambaqui. In the toxicity test with PZQ, the main hematological alterations were those related to the time of exposure, with reduction in hematocrit, hemoglobin and MCHC, in addition to a significant dose and time-dependent alteration in blood glucose. No mortality was observed, except three fish exposed to 50 mg L−1 for 24 h. No treatment removed 100% of the parasites from the gills of tambaqui; however, the lowest concentrations of PZQ were more effective against Anacanthorus sp., removing 61.8% at 12.5 mg L−1. With these results, it is possible to consider key moments during tambaqui farming to apply the medication to prevent disease outbreaks.