Previous research has shown that the effects of different drugs on the growth rate of blow flies vary by species, which can influence the determination of the minimum post-mortem interval (PMImin​). In this study, we assessed how morphine affects the development, growth, and survival rate of Chrysomya megacephala (Fabricius, 1794), a forensically important blow fly species, and whether these effects are dose dependent. We also measured morphine concentrations in insect samples (3rd -instar larvae and puparial cases) and in various post-mortem tissues of rabbits (cardiac blood, skeletal muscle, and liver). Rabbits were administered two lethal doses of morphine hydrochloride (5 and 10 mg/kg), then sacrificed, and tissue samples were collected for toxicological analysis. At a controlled temperature of 37 °C, newly hatched larvae (1-2 h old) were placed on the rabbit carcasses and randomly sampled at 12 h regular interval (12-132 h) to determine larval development time, puparial development time, larval mortality, larval length, and larval weight. Changes in these insect parameters led to an underestimation of PMImin by up to 15.92 h if the presence of morphine was not considered. Moreover, morphine concentrations in all insect samples (180 and 223 ng/g in the 3rd instar larvae; 40 and 45 ng/g in the puparial cases after treatment with 5 and 10 mg/kg of morphine hydrochloride, respectively) and rabbit tissues (1129 and 2564 ng/g in the cardiac blood; 1777 and 2823 ng/g in the liver; 825 and 1070 ng/g in the skeletal muscles after treatment with 5 and 10 mg/kg of morphine hydrochloride, respectively) were measured using high-performance liquid chromatography system equipped with a binary pump and UV detector. Morphine can affect the development of different life cycle stages of Ch. megacephala, highlighting the importance of accounting for its presence when determining PMImin ​ in such conditions. The data obtained could aid in criminal investigations. Although a concentration-dependent effect has been discussed, the use of whole animal carcasses may lead to uneven distribution of the drug, and concentrations in the areas from which larvae are feeding may differ significantly due to ante-mortem processes in the body.