Flexible health spending accounts became far more flexible last week, increasing the chance that more employees will save money by using them.

The accounts let employees pay medical bills with pre-tax dollars, but only about 20 percent take advantage of the option. That’s at least partly a result of the use-it-or-lose-it rule that forces participants to forfeit money left in their accounts at the end of the year. Employers keep the forfeited amounts, and many use the money to pay the costs of managing the program.

Workers can put up to $2,500 into a flex account and then withdraw it to pay for medical expenses, such as deductibles, copays, prescriptions and other out-of-pocket medical expenses. Employees’ tax bills are reduced because they don’t pay taxes on the money they put in the account. Before this year, the limit was $5,000 and employees could use the funds for many over-the-counter medical purchases; that was trimmed to save tax funds, helping to pay for the Affordable Care Act.

The difficulty, though, is in guessing how much the medical bills would be. Putting too much in the account risked losing money and putting in too little sacrificed some of the tax advantage. The government said it received more than 1,000 comments from employers and employees about how hard it was to guess at the right figure. Employees ended up scrambling at the end of the year to use the funds.

The change announced last week by the U.S. Treasury Department said employers may now allow employees to carry over $500 from one year to the next. Employers were already allowed to grant a grace period until March 15 for the money’s use. The new rule permits either the grace period or the carryover, but not both. Also, employers are permitted, but not required, to make changes.

The $500 amount was chosen because most forfeitures have been less than that. About one in four participants were estimated to have some amount left over and lost.

The change means more employees are likely to see benefit in the accounts and make good use of them. It’s a small bright spot in a time when implementation of the Affordable Care Act is causing confusion on the subject of paying for health care.