Sleek and featherweight in metallic black or navy blue, Juul-brand electronic cigarettes have become a fashion — you could say a contagion — among high-school students across America. Easily mistaken for an ordinary flash drive, the gadgets let kids “Juul” without notice in hallways or school cafeterias and conveniently recharge on their laptops.

The U.S. Food and Drug Administration is failing in its responsibility to regulate e-cigarettes. Tobacco in any form imperils children’s lives.

In 2016, 1.7 million high-school students (11.3 percent) used e-cigarettes, plus a half million middle-school students. Beguiled by unregulated ads and fruit flavorings, and undeterred by the federal prohibition on sales to minors, children take in as much nicotine from Juuls as come from combustible cigarettes. Once addicted to vaping, research suggests, they may be more easily drawn to old-fashioned smoking.

Vapers suffer nicotine’s ill effects on blood pressure and heart rate and expose themselves to any number of other toxic chemicals.

FDA Commissioner Scott Gottlieb says he hopes e-cigarettes might be useful in helping smokers quit. But there’s far too little evidence to support that idea. And even if e-cigarettes help a few people escape tobacco, they attract so many teenagers that their net effect is decidedly harmful.

The FDA is belatedly considering limiting or banning e-cigarette flavorings. But Gottlieb has postponed any further regulations. The American Academy of Pediatrics and several other health groups have filed suit to challenge the FDA’s delay.

Litigation shouldn’t have been necessary. The agency ought to move quickly to restrict e-cigarette advertising and online sales, and to impose testing and labeling rules so consumers know what dangerous ingredients e-cigarettes contain. Further delay will prolong the myth that e-cigarettes are benign, will give a malignant habit time to spread and will destroy the health of millions of American children. Just because there’s no smoke doesn’t mean cigarettes can’t kill you.