On the question of whether a soda tax can actually reduce the amount of sugary drinks people consume, a new study finds the resounding answer is “yes.”

In November 2014, Berkeley, California, voters passed the nation’s first tax on sugar-sweetened beverages in an effort to reduce their impact as a major contributor to chronic diseases such as obesity and type 2 diabetes. The small tax was just a penny-per-ounce on sodas, energy and sports drinks, fruit-flavored drinks, and sweetened water, coffee and teas. But according to researchers, that small tax is already having a big impact. In a study published earlier this week in the American Journal of Public Health, researchers found that just a few months after the tax went into effect, sugar-sweetened beverage consumption went down by a whopping 21 percent in Berkeley, while such consumption increased in comparison cities.

To conduct the study, researchers surveyed a total of nearly 3,000 residents across Berkeley, San Francisco and Oakland both before and after the soda tax was implemented. (San Francisco also attempted to pass a soda tax in 2014, but voters rejected the measure.) The surveys were focused in low-income and minority neighborhoods, where people are “more likely to consume (sugar-sweetened beverages) and suffer related consequences,” the study stated. Survey participants were asked how often they drank sugary drinks as well as how often they drank plain water from a bottle or tap.

Here’s what the study found: In low-income neighborhoods in Berkeley, sugar-sweetened beverage consumption plummeted 21 percent over a one-year period from before the tax to after the tax, while such consumption actually increased by 4 percent in comparison neighborhoods in Oakland and San Francisco. More specifically, soda drinking went down by 26 percent in Berkeley, while increasing by 10 percent in the comparison cities, and consumption of sports drinks went down by 36 percent in Berkeley, while increasing 21 percent in comparison cities. In addition, water consumption in Berkeley increased by 63 percent over the study period, but only by 19 percent in San Francisco and Oakland.

The study noted that the Berkeley result may not be entirely due to higher prices, but “could also reflect effects of the campaign surrounding the tax, which may have shifted social norms and thus reduced consumption.” Study authors Jennifer Falbe, Hannah Thompson, Christina Becker, Nadia Rojas, Charles McCulloch and Kristine Madsen write:

(A sugar-sweetened beverage) excise tax is one of the few public health interventions expected to reduce health disparities, save more money than it costs, and generate substantial revenues for public health programs. Already, Berkeley City Council has allocated $1.5 million to fund programs to reduce (sugar-sweetened beverage) consumption and address obesity for the 2016–2017 fiscal year. …If impacts in Berkeley persist, and evidence from other cities passing (sugar-sweetened beverage) taxes corroborate our findings, widespread adoption of (sugar-sweetened beverage) excise taxes could have considerable fiscal and public health benefits.

In June of this year, Philadelphia joined Berkeley in passing a soda tax. According to the Philadelphia City Council, the 1.5 cents-per-ounce tax on sodas and other sweetened beverages will raise $91 million over the next year to fund quality pre-kindergarten expansion, community schools, parks and recreations centers, and help pad Philly’s General Fund.

The Centers for Disease Control and Prevention reports that U.S. medical costs related to obesity reached $147 billion in 2008, while the bill for medical care and lost wages and work associated with diabetes is $245 billion.

To request a full copy of the new soda tax study, visit the American Journal of Public Health. For more on the Berkeley vs. Big Soda campaign, click here.

Kim Krisberg is a freelance public health writer living in Austin, Texas, and has been writing about public health for nearly 15 years.