A new study from Yale University warns that cities may need to think twice before banning flavored tobacco products, including flavored vape liquids that contain nicotine. That’s because San Francisco’s 2018 ban on flavored tobacco products appears to have had the opposite of its intended effects – rather than reducing nicotine use among teenagers, evidence suggests that more teenagers are being encouraged to use traditional tobacco cigarettes .

The idea behind the ban on flavored tobacco – which included menthol – was that teenagers are more likely to vape or use cigarettes when they have tasty flavors like desserts or fruits. The ban, proponents argued, would remove the appeal of these products, which would hopefully lead to a drop in tobacco use among teenagers … but it didn’t.

According to the new Yale study, teenagers in San Francisco were more likely to smoke normally flammable cigarettes after the taste ban doubled, contrary to trends seen elsewhere. Without the flavors, the researchers said, teenagers may have no incentive to choose electronic cigarettes over their flammable alternative, driving them to packs of cigarettes rather than vape pods.

This is a problem for obvious reasons: while vapes are not safe, research has shown that they are significantly less harmful than regular tobacco cigarettes. The study found that banning flavored vape products for cities aimed at improving public health can backfire by leading young users to the most harmful option.

The researchers suggest that cities looking for alternatives should consider restricting tobacco product sales to stores that only allow adults 21 and older. This would reduce teenagers’ exposure to the products in common places they might visit, such as gas stations, and make it more difficult for teenagers to acquire.