This study investigates the succession of insects and the length of the decomposition of rabbit carcasses injected with heroin. In total, 24 rabbits (2–2.5 kg, Oryctolagus cuniculus domesticus L.) were used in the study. Twelve carcasses were exposed to insect activity in winter (January and February) and 12 in summer (July and August) in Riyadh, Saudi Arabia. Three replicate experiments were conducted in each season. For each experiment, three rabbits were injected with varying doses of heroin, and the fourth rabbit was used as a control, being injected only with saline solution. Insects belonging to the order Diptera, Coleoptera, and Hymenoptera were attracted to the carcasses. In both of the investigated seasons, heroin appeared to delay the decomposition process but did not have a significant impact on the number of insects. In general, the number of flies in both seasons was substantially higher than that of beetles and ants. During the summer, flies were more attracted to treated carcasses with a higher dose of heroin. Some flies such as Musca domestica L. (Diptera: Muscidae) and Chrysomya albiceps Wiedemann (Diptera: Calliphoridae) and some beetles such as Pimelia boyeri Solier (Coleoptera: Tenebrionidae) were represented on the carcasses by different developmental instars (larvae, pupae, and adults). Overall, heroin has not potential effects on the insect succession patterns of insect, however, influences the decomposition of rabbit carcasses. These findings are important while using insects in medicolegal investigation, since heroin is one of the common poisons used as an addictive agent.