Which statement about therapeutic index is true?

Which statement about therapeutic index is true?

• A. TI equals ED50/LD50
• B. TI is a rough estimate with no relation to safety
• C. A drug with a small TI is safer
• D. The larger the TI, the safer the drug

Answer: (D)

The safety margin is what the therapeutic index measures. It compares the dose that produces toxicity (or lethal effects) to the dose that produces the desired therapeutic effect. A larger ratio means you can achieve the therapeutic effect well before hitting toxic levels, giving a wider safety margin. That’s why a larger therapeutic index is associated with a safer drug. In practice, the index is defined as TD50 or LD50 over ED50, with ED50 being the dose for the desired effect and TD50/LD50 the dose for toxicity or lethality. It’s a useful, though imperfect, comparison tool across drugs. A small index means the toxic dose is close to the effective dose, implying less safety.

The safety margin is what the therapeutic index measures. It compares the dose that produces toxicity (or lethal effects) to the dose that produces the desired therapeutic effect. A larger ratio means you can achieve the therapeutic effect well before hitting toxic levels, giving a wider safety margin. That’s why a larger therapeutic index is associated with a safer drug.

In practice, the index is defined as TD50 or LD50 over ED50, with ED50 being the dose for the desired effect and TD50/LD50 the dose for toxicity or lethality. It’s a useful, though imperfect, comparison tool across drugs. A small index means the toxic dose is close to the effective dose, implying less safety.