Why does the fact that a drug has a low therapeutic index mean that the physician must use greater care in its administration? (2 marks)
A. Drugs with a low therapeutic index can cause toxicity at a high ED50.
B. Drugs with a low therapeutic index can only cause toxicity over a small range of drug doses.
C. Drugs with a low therapeutic index can cause irreversible effects after they are administered.
D. Drugs with a low therapeutic index do not have therapeutic effects until the administered dose approaches the toxic dose.
E. Drugs with a low therapeutic index have a greater degree of safety when administered at therapeutic doses.
The correct answer and explanation is:
The correct answer is B. Drugs with a low therapeutic index can only cause toxicity over a small range of drug doses.
Explanation:
The therapeutic index (TI) is a measure used to evaluate the safety of a drug. It is calculated as the ratio between the median toxic dose (TD50), the dose at which 50% of individuals exhibit toxicity, and the median effective dose (ED50), the dose at which 50% of individuals experience the therapeutic effect. The formula is typically expressed as:
TI = TD50 / ED50
A low therapeutic index means that the difference between the effective dose (the dose that produces the desired therapeutic effect in 50% of the population) and the toxic dose (the dose that produces harmful side effects in 50% of the population) is small.
This narrow margin between the effective and toxic doses means that there is little room for error in dosage. Administering even slightly higher doses than what is required for the therapeutic effect can cause severe toxicity, while doses too low may fail to produce the desired therapeutic effect.
Drugs with a low therapeutic index require careful monitoring to avoid toxicity. Physicians must exercise caution when prescribing such drugs and often perform frequent monitoring of drug levels in the blood to ensure that the concentration remains within the therapeutic range. This is particularly important for drugs that are used for chronic conditions, as maintaining appropriate dosing over time is critical to avoid long-term adverse effects.
In contrast, drugs with a high therapeutic index have a wide margin between the effective and toxic doses, making them safer and easier to administer with less frequent monitoring.