Which of the following is a characteristic of narrow therapeutic index drugs?

Narrow therapeutic index drugs are medications where small changes in dose or blood concentration can lead to therapeutic failures or adverse effects. Therefore, one of their key characteristics is the necessity for a tighter limit on assay variation between dosage units. This means that the consistency and accuracy in measuring the amount of the drug within each dosage unit must be stricter than for drugs with a broader therapeutic index. This is crucial because even slight variations can significantly impact a patient's safety and efficacy of treatment, underscoring the importance of precise monitoring and manufacturing standards for these types of medications.

While higher risks of side effects and lower dosing frequency might be considerations in some contexts, they are not defining characteristics specifically associated with the classification of narrow therapeutic index drugs. Extended expiration dating typically reflects the stability of the drug over time but does not inherently correlate with therapeutic index considerations. Hence, a tightly controlled assay variation is the hallmark trait that distinguishes these drugs in clinical practice.