Rice is a monocotyledonous plant in the Poaceae family. The cultivation of this cereal is prevalent in Madagascar. It is the staple food of the Malagasy people. This study concerns the determination of the lethal dose 50 of the X243 variety rice root by administering three different doses: 100 mg/kg, 500 mg/kg, and 2048 mg/kg of the body weight of a Swiss-type albino mouse. The doses administered gave two values of LD50. The logarithmic value is 1258,93 mg/kg body weight, and the linear value is 1495,24 mg/kg. These values are very close. Consequently, the roots of the X243 variety rice are toxic. Moreover, phytochemical screening shows the presence of alkaloids, tannins, polyphenols, and cyanogenic compounds. There’s no doubt that cyanogenetic compounds are extremely poisonous and responsible for poisoning. Our results confirm the presence of poison in the rice roots already known to the population.