Why is close monitoring necessary for drugs with a narrow therapeutic range?

Close monitoring is necessary for drugs with a narrow therapeutic range because these drugs have a small window between their therapeutic dose and toxic dose. This means that small changes in the drug's concentration in the blood can lead to ineffective treatment or significant toxicity. For instance, if the drug level rises just slightly above the therapeutic range, it can cause adverse effects that may be harmful or even life-threatening. Therefore, careful monitoring helps ensure that the patient remains within this narrow range, optimizing efficacy while minimizing the risk of toxicity.

In the context of the other options, drugs being less effective, not commonly prescribed, or having high rates of metabolism do not fundamentally relate to the critical need for close monitoring. The primary concern revolves around the potential for toxicity if drug levels are not carefully managed.