Therapeutic index is defined as the ratio of the dose to produce toxic/lethal effects to the dose to produce a therapeutic effect.

• A. The ratio of the therapeutic dose to the toxic dose
• B. The ratio of the dose producing toxicity to the dose producing a therapeutic effect
• C. Toxic dose divided by therapeutic dose
• D. The ratio of the maximum tolerated dose to the therapeutic dose

Answer: (B)

The main idea here is safety margin: how much of a drug you can give before it becomes toxic compared with the dose that provides benefit. The therapeutic index is expressed as the ratio of the dose that produces toxicity (or lethal effects) to the dose that produces a therapeutic effect. That means it’s comparing the harmful potential to the beneficial effect.

So the correct description is the ratio of the toxic/lethal dose to the therapeutic dose. A larger ratio means a wider safety margin and a safer drug; a small ratio means the drug has a narrow margin between effective and toxic effects.

The other ways of framing it aren’t the standard TI. For example, a ratio of therapeutic dose to toxic dose would be the inverse of the typical TI and can be misleading for safety assessment, and using the maximum tolerated dose divided by the therapeutic dose refers to a different concept related to safety margins but isn’t the conventional therapeutic index.