Why does the fact that a drug has a low therapeutic index mean that the physician must use greater care in its administration?
The correct answer and explanation is:
Correct Answer:
A low therapeutic index means that the difference between the effective dose and the toxic dose is small, so the physician must use greater care to avoid causing harm to the patient.
300-Word Explanation:
The therapeutic index (TI) is a measure of a drug’s safety and is defined as the ratio between the toxic dose (TD₅₀) and the effective dose (ED₅₀). Specifically: Therapeutic Index=TD50ED50\text{Therapeutic Index} = \frac{TD_{50}}{ED_{50}}
- ED₅₀ is the dose that produces the desired therapeutic effect in 50% of the population.
- TD₅₀ is the dose that causes toxicity in 50% of the population.
A low therapeutic index means this ratio is small, indicating that the effective and toxic doses are close together. In such cases, even a small increase in dose—whether due to physician error, patient noncompliance, or drug interactions—can push the patient from a therapeutic range into a toxic range.
Examples of drugs with low therapeutic indices include:
- Digoxin (used for heart conditions)
- Warfarin (a blood thinner)
- Lithium (used in bipolar disorder)
- Theophylline (used for asthma and COPD)
Because of this narrow margin of safety, physicians must exercise greater care in several ways:
- Precise dosing based on patient weight, age, kidney/liver function
- Close monitoring of blood drug levels (therapeutic drug monitoring)
- Adjusting doses carefully in response to lab results or side effects
- Educating patients on adherence and avoiding drug interactions
In summary, a low therapeutic index demands increased vigilance from physicians to ensure that the drug provides benefit without causing harm. Failure to do so may result in underdosing (ineffectiveness) or overdosing (toxicity), both of which can have serious or even fatal consequences.