When does the additional income-based monthly insurance premium start to be applied?

The additional income-based monthly insurance premium, also known as the "income-based monthly premium" (소득월액보험료), typically begins to be applied starting from November of each year.

This is because the National Health Insurance Service (NHIS) uses the income data reported to the National Tax Service (NTS) for the comprehensive income tax filing. The process generally follows these steps:

1. May: Comprehensive income tax filing for the previous year is completed.
2. October: The NTS transmits the income data to the NHIS.
3. November: The NHIS recalculates the insurance premiums based on the new income data, and the adjusted premiums, including any additional income-based monthly premiums, are applied starting from this month.

Therefore, the premiums applied from November of a given year reflect the income earned in the preceding year.