Women are saving a lot of money as a result of a health law requirement that insurance cover most forms of prescription contraceptives with no additional out-of-pocket costs, according to a study released Tuesday. But the amount of those savings and the speed with which those savings occurred surprised researchers.

The study, in the July issue of the policy journal Health Affairs, found that the average birth control pill user saved $255 in the year after the requirement took effect. The average user of an intrauterine device (IUD) saved $248. Those savings represented a significant percentage of average out-of-pocket costs.

“These are healthy women and this on average is their No. 1 need from the health care system,” said Nora Becker, an MD-PhD candidate at the University of Pennsylvania and lead author of the study. “On average, these women were spending about 30 to 44 percent of their total out of pocket (health) spending just on birth control.”

The study looked at out-of-pocket spending from nearly 800,000 women between the ages of 13 and 45 from January 2008 through June 2013. For most plans, the requirement began Aug. 1, 2012, or Jan. 1, 2013. So-called “grandfathered” health plans, those that have not substantially changed their benefits since the health law was passed in 2010, are exempt from the mandate, as are a small subset of religious-based plans.

Becker said that while making birth control substantially cheaper may not increase the number of women who use it, the new requirements could well shift the type of birth control they use to longer-acting, more effective methods like the IUD. “If prior to the ACA a woman was facing $10 to $30 a month for the pill but hundreds of dollars upfront for an IUD and now both are free, we might see a different choice,” she said.

Researchers also found that while out-of-pocket spending dropped dramatically for most types of prescription contraceptive methods — “the majority of women were paying nothing by June 2013” –spending barely budged for the vaginal ring or hormonal patch.

That could be because under the original rules, many insurers declined to make the ring or patch free, since, like pills, they are essentially hormone delivery methods.  Earlier this year, the Obama administration issued a clarification saying that while insurers do not have to offer every brand of every method, they do have to cover at least one product in each category, including rings and patches.