LD50 is expressed as a dose per body weight; what is the typical standard unit?

• A. mg/kg
• B. g/L
• C. mg/L
• D. kg/mg

Answer: (A)

LD50 is the amount of a substance that would kill 50% of a test population, expressed relative to the subject’s body weight so the dose scales with size. The standard unit is milligrams of substance per kilogram of body weight (mg/kg). This lets you compare toxicity across animals of different weights because a given mg/kg dose corresponds to a proportionally appropriate total amount for each animal. Units like g/L or mg/L are concentrations in a solution, not doses per body weight, and kg/mg would invert the relationship and isn’t used to express LD50.