When a promotional interest rate period on a credit card ends, the regular APR (Annual Percentage Rate) is applied to any remaining balance. Many credit card issuers offer promotional rates, such as 0% APR for a set number of months, to attract new cardholders or encourage balance transfers. However, it's essential to understand what happens after the promotional period ends. If you haven't paid off your balance in full, the remaining balance will start accruing interest at the card's regular APR, which could be significantly higher.

It's important to note that some credit cards may also apply retroactive interest if the balance isn't paid off by the end of the promotional period. This means that interest could be charged on the entire balance from the date of the purchase, not just the remaining amount. To avoid this, it's crucial to pay off the balance in full before the promotional period ends. If you think you won't be able to do so, consider transferring the balance to another card with a lower interest rate to avoid high charges.