House Speaker Nancy Pelosi on Wednesday voiced her opposition to e-cigarette company Juul and its efforts to pass a San Francisco ballot measure, Proposition C, that would overturn a sales ban on e-cigarettes and allow the company to continue selling the products in San Francisco with some new restrictions.

“No on C. We can’t afford this brazen special-interest attempt to addict our children to cigarettes,” Pelosi said at a San Francisco Democratic Party ceremony honoring her with a lifetime achievement award. “With all the unknown short-term and long-term consequences of e-cigarettes, we cannot let corporate special interests buy themselves this proposition. So, children, teachers, parents, leaders, policymakers — say no to Juul, no on C.”

Juul has spent $4.3 million to back Prop. C — more than other San Francisco ballot initiative backers combined for the Nov. 5 election. The contributions have gone to a committee created to back the ballot measure, called the Coalition for Responsible Vaping Regulation.

Prop. C would override legislation passed by the Board of Supervisors in June that prohibits the sale of e-cigarettes until they are reviewed by the federal Food and Drug Administration.

Prop. C would instead allow the continued sale of e-cigarettes in the city, with some new regulations. It would require online retailers that sell e-cigarettes to San Francisco residents to apply for a permit; require brick-and-mortar stores to use new age-verification technology to make sure customers are at least 21, the legal age to buy tobacco in California; and limit each customer to two devices and five packs (20 pods total) of nicotine cartridges in a single transaction in stores, and two devices and 60 milliliters (about 86 pods) per month online.

Juul has drawn widespread criticism from legislators, parents and public health officials for its role in the youth vaping epidemic. But the company, which analysts estimate has more than 75% of the U.S. e-cigarette market, has maintained that its vaping products are meant to help adult cigarette smokers switch to vaping. E-cigarettes are considered less harmful because they don’t emit the same combustible carcinogens as traditional cigarettes. Many adult cigarette smokers say that vaping has helped them breathe more easily, cough less and feel better overall.

Juul, headquartered in San Francisco, designed an e-cigarette that is more accessible, discreet and easier to use than many older vaping products. But the same qualities, health researchers say, have made the product attractive to teens — whose still-developing brains are especially susceptible to becoming addicted to the nicotine in e-cigarette vapor. Juul took steps last year to try to curb use of its products among youth, announcing it would stop selling its popular flavored nicotine like mango and creme in stores.

Researchers have yet to determine the long-term health effects of vaping, in part because the products have been widely sold only for about a decade. Some recent and preliminary studies show regular e-cigarette use is associated with greater risk of heart attack and stroke, and researchers have observed that vaping seems to trigger a reaction from the respiratory system that leads to coughing, wheezing and worsening of asthma.

“We agree with Speaker Pelosi that no one under the age of 21 should ever vape,” Nate Allbee, a spokesman for the Coalition for Responsible Vaping Regulation, which is backed by Juul, said in a statement. “Vaping is only for adults trying to quit smoking. That’s why our initiative creates the strongest regulations on any adult product in San Francisco. Stronger then the current regulations on cigarettes, alcohol, and even marijuana.”

Allbee said the coalition “respectfully disagrees” with the city’s approach to address youth vaping.

“San Franciscans know that bans don’t work, and telling adults that they can’t vape and should go back to smoking cigarettes is bad public health policy,” he said.

Catherine Ho is a San Francisco Chronicle staff writer. Email: cho@sfchronicle.com Twitter: @Cat_Ho