Many credit card customers don’t realize the short- and long-term effects their cards’ APR (Annual Percentage Rate) can have on their finances. But it’s a crucial detail for credit card holders to understand. That’s because, if you carry a debt on your card from month-to-month, your APR determines how much interest you pay on purchases. A low APR is ideal – but it can increase seemingly out of the blue.

There are several factors that impact your APR.  Some might be influenced by card holder behaviors, while others aren’t. It’s important to understand the terms surrounding your APR so you’re not surprised by an APR increase down the road, long after you’ve opened new credit.  Here are five possible reasons your APR could go up.

Introductory or Promotional Rate is Expiring

Many consumers take advantage of promotions like low interest rates when they first open a card. But low APR offers don’t last forever – these promotional rates usually last six to 12 months after opening the account. Read the fine print so you aren’t blindsided after securing a new card.

If you’ve opened a low-APR card and transferred a balance in the hope of saving money on interest, it’s especially important to pay off that balance transfer before the interest rate goes up.

Late Payments

Missed a credit card payment by 60 days or more? In that case, your card issuer may impose a higher APR. It might take a while for this high interest rate to go back down.

If you consistently face challenges making your credit card payments on time, consider a debt management plan. These plans allow you to make one consolidated monthly payment and obtain lower interest rates so you can pay off your debt more quickly.

A Credit Score Drop

Credit card issuers intermittently monitor your credit score. If you experienced a substantial drop in your credit score, your card issuer might respond by increasing your APR. If your score improves afterwards, though, the issuer must consider reducing your rate.

Variable APR

Some credit cards are accompanied by a variable APR rate, meaning it changes as interest rates in general change. Many companies set rates linked to the prime rate.

How Long You’ve Had the Card

Legally, credit card companies can’t raise your rate until you’ve had the card for a year or longer and they must give you 45 days’ notice. There are a few exceptions, including a variable APR, a 60-day delinquency or a promotional rate that’s ending. But after one year, the card issuer can raise your rate if they want to — even if you have consistently  paid on time and maintained good credit overall .

The best way to prevent an expensive hike in APR is by paying off the full balance when it’s due to avoid carrying debt from month-to-month.