What is TD50?

• A. Toxic dose in 50% of the population
• B. Therapeutic dose in 50% of the population
• C. Peak blood level in 50% of population
• D. Onset time for 50%

Answer: (A)

TD50 is the dose that produces a toxic effect in 50% of the population. It’s a toxicology metric used to gauge a drug’s safety margin by comparing how much is needed to cause toxicity versus how much is needed to achieve a therapeutic effect (the latter is ED50). It’s different from peak blood level, which is a concentration, and from onset time, which is how quickly the drug starts working. It’s also distinct from LD50, which is the lethal dose for 50% of the population. For example, if the therapeutic effect occurs at 10 mg (ED50) and toxicity appears around 200 mg (TD50), the therapeutic index TD50/ED50 = 20 indicates a relatively wide safety margin.