San Francisco’s ban on flavored vaping products may have led to an increase in teens’ use of regular cigarettes, a new study suggests. The city’s ban was passed in 2018 and fully implemented in 2019.

Researchers at the Yale School of Public Health compared smoking rates of high school students in San Francisco and other school districts between 2011 and 2019. They found that before the ban was implemented, past-month smoking rates in San Francisco and other school districts were declining. After the ban was fully implemented, smoking rates in San Francisco teens rose, while those in comparison districts continued to decline.

The odds that high school students in San Francisco would smoke regular cigarettes doubled compared to school districts without a ban on flavored vaping products, HealthDay reports.

“These findings suggest a need for caution,” lead researcher Abigail Friedman said in a news release. “While neither smoking cigarettes nor vaping nicotine are safe per se, the bulk of current evidence indicates substantially greater harms from smoking, which is responsible for nearly one in five adult deaths annually. Even if it is well-intentioned, a law that increases youth smoking could pose a threat to public health.”