Which statement best describes a high TI?

Which statement best describes a high TI?

• A. It indicates a small margin between effective and toxic doses.
• B. It implies the drug is ineffective.
• C. It indicates a wide margin between effective and toxic doses.
• D. It indicates strong adverse effects at all doses.

Answer: (C)

A high therapeutic index means there is a wide safety margin between the dose that achieves the desired effect and the dose that causes toxicity. This index is often expressed as the ratio of a toxic (or lethal) dose to an effective dose (TD50/ED50 or LD50/ED50). When this ratio is large, the toxic dose is much higher than the effective dose, so the drug can be used with less risk of toxicity at the therapeutic dose. That’s why the statement describing a wide margin between effective and toxic doses is the best description. Conversely, a small margin would indicate a low therapeutic index and higher risk of toxicity at therapeutic doses. The idea is not that the drug is ineffective or that adverse effects occur at all doses; it's about how far apart the safe and harmful dose ranges are.

A high therapeutic index means there is a wide safety margin between the dose that achieves the desired effect and the dose that causes toxicity. This index is often expressed as the ratio of a toxic (or lethal) dose to an effective dose (TD50/ED50 or LD50/ED50). When this ratio is large, the toxic dose is much higher than the effective dose, so the drug can be used with less risk of toxicity at the therapeutic dose. That’s why the statement describing a wide margin between effective and toxic doses is the best description. Conversely, a small margin would indicate a low therapeutic index and higher risk of toxicity at therapeutic doses. The idea is not that the drug is ineffective or that adverse effects occur at all doses; it's about how far apart the safe and harmful dose ranges are.