Local subscribers whose health insurance burden has changed due to income fluctuations should carefully consider 'when' they apply for a premium adjustment. Depending on the application date, the month in which the adjusted premium is first reflected may differ.

■ A one-day difference determines when the premium applies

Suppose a local subscriber whose total income this year has decreased significantly compared to last year applies for a premium adjustment with the National Health Insurance Service (NHIS) on September 30. In principle, the adjusted premium applies from October, the month following the application, through the end of the year. If the application is made on October 1, the effective date is likewise October. This is because an exception rule allows adjusted premiums to apply from the current month for applications received on the first day of each month.

As a result, both a September 30 and an October 1 application result in the adjusted premium being reflected for a total of three months: October, November, and December.

However, if the application is made on or after October 2, the situation changes. In principle, the adjustment applies from November, the month following the application. Considering the time remaining until year-end, the actual period during which the adjusted premium applies is only two months: November and December. Although the application dates differ by just one day, the number of months covered can differ.

■ 2026 adjustment eligibility and application periods

For 2026, those eligible to apply for premium adjustment include ▲local subscribers whose income has decreased or increased, and ▲employees subject to premiums on income other than wages. The basic adjustment period runs from the month following the application date through December of that year.

The application period varies depending on the grounds for adjustment, so it must be confirmed in advance. According to NHIS guidance, if the grounds for adjustment fall under 'income amount verification,' the application period runs from July through October each year. Thus, as of late September, the opportunity to apply remains until October.

■ Separate application standards at year-end and the start of the year

Separate standards also apply at year-end and the beginning of the year. For November, December, and January of the following year, if an application is submitted between the first day of that month and the premium payment deadline, the adjusted premium applies from the month of application rather than the following month.

■ Settlement follows even after adjustment, with refunds or additional charges possible

However, receiving a premium adjustment does not mean the amount is finalized. A settlement procedure is subsequently carried out based on actual income data confirmed by the National Tax Service and other sources. If the income confirmed during settlement is lower than the amount reported at the time of adjustment, the premium difference may be refunded. Conversely, if the actual confirmed income is higher, additional premiums may be imposed. Therefore, adjustment applicants should also review the results of the post-settlement.

■ NHIS refund inquiry channels and things to check

National Health Insurance refunds can be checked through official channels provided by the NHIS, including its website, mobile app, and customer center.

Before applying, one should first confirm whether the grounds for adjustment fall under 'income amount verification.' If so, check whether the application period extends through October. Additionally, consider that depending on income confirmed later, the already adjusted premium may be recalculated, resulting in a refund or additional charges.

In particular, for those seeking to have the adjusted premium applied from October if possible, the application date is critical. Applications received by October 1 can be applied from October's premium, but applications made on or after October 2 are, in principle, reflected from November's premium, potentially reducing the application period within the year by one month.

[This article is policy information based on NHIS guidance, and detailed standards and schedules may change according to NHIS policy. Individual applicability and refund eligibility must be confirmed through the NHIS. This article was partially assisted by AI.]