Based on the case, which factor most likely contributed to the subtherapeutic phenytoin level?

Based on the case, which factor most likely contributed to the subtherapeutic phenytoin level?

• A. Not taking the medication regularly.
• B. Increased metabolism due to liver disease.
• C. Recent alcohol use.
• D. Drug interaction with antacids.

Answer: (A)

Phenytoin has a narrow therapeutic range and its levels in the blood depend heavily on consistent dosing. When a patient does not take the medication regularly, there are gaps between doses and trough levels fall below the therapeutic range, increasing the risk of seizures. This direct link between adherence and plasma concentration makes not taking the medication regularly the most likely reason for a subtherapeutic phenytoin level in the case. Other factors can influence phenytoin levels, but they don’t fit as neatly with a pattern of subtherapeutic levels driven by inconsistent dosing. Liver disease often reduces metabolism, which can raise levels rather than lower them; alcohol use can alter levels depending on timing and duration, but it’s less predictable as a sole cause of a sustained subtherapeutic level; and antacids can decrease absorption if taken with the dose, but that explains a timing-related change rather than a chronic pattern of nonadherence.

Phenytoin has a narrow therapeutic range and its levels in the blood depend heavily on consistent dosing. When a patient does not take the medication regularly, there are gaps between doses and trough levels fall below the therapeutic range, increasing the risk of seizures. This direct link between adherence and plasma concentration makes not taking the medication regularly the most likely reason for a subtherapeutic phenytoin level in the case.

Other factors can influence phenytoin levels, but they don’t fit as neatly with a pattern of subtherapeutic levels driven by inconsistent dosing. Liver disease often reduces metabolism, which can raise levels rather than lower them; alcohol use can alter levels depending on timing and duration, but it’s less predictable as a sole cause of a sustained subtherapeutic level; and antacids can decrease absorption if taken with the dose, but that explains a timing-related change rather than a chronic pattern of nonadherence.