Beginning in 1998, Washington, DC, wasn’t permitted to use its funds to support needle exchanges. Changing that policy spared an estimated 120 drug users from HIV infection over a two-year period, per a study by George Washington Universitypublished today in AIDS and Behavior. The change came in late 2007, and USA Today reports that the needle exchange’s initial two-year period cost the city’s health department $1.3 million.

But the avoided cases saved far more than that: A lifetime of HIV treatment for 120 people works out to $45.6 million, for a net savings of $44.3 million, per the researchers, who point out that “many injection drug users are covered by publicly funded health plans.” How lead author Monica Ruiz sees it: “Our study adds to the evidence that needle exchange programs not only work but are cost-effective investments in the battle against HIV.” In addition to allowing users to swap used syringes for fresh ones, the DC program offered free HIV tests and condoms.

While the programs are gaining support across the country (by USA Today’s count there are about 220 of them nationwide), they are not without their opponents.

“My fundamental concern is that many of the needle exchanges aren’t going after the core problem, which is drug abuse,” says the chair of the Institute on Global Drug Policy.

(DC’s program does provide referrals to those looking for treatment.) The federal government’s ban on spending on such programs has remained in place since 1998. (Meanwhile, an insurer has just announced a major HIV breakthrough.)