If a chemical has an LD50 of 200 mg/kg in rats, what does this mean?

• A. A single injection of 200 mg/kg would kill all rats
• B. On average, 200 mg/kg would kill 50% of rats
• C. 200 mg/kg would be toxic but not lethal
• D. It is safe at 200 mg/kg in all species

Answer: (B)

LD50 means the median lethal dose—the amount of a chemical per kilogram of body weight that, under defined test conditions, would kill about 50% of a population. So an LD50 of 200 mg/kg in rats indicates that if groups of rats are exposed to that dose, roughly half would die within the observation period. It does not guarantee that all rats will die, and it doesn’t say anything about safety in other species or about effects at lower doses. The other statements misinterpret LD50: it is a lethal dose for a population under specific conditions, not a universal killer, and not a statement about safety across species.