Food and Drug Administration Commissioner Scott Gottlieb. (J. Scott Applewhite/AP)

ONE OF the Trump administration’s most competent, careful and effective senior officers will soon leave government. At stake is the health of millions of Americans.

Food and Drug Administration Commissioner Scott Gottlieb will depart within a month. He heads one of the nation’s most powerful government agencies, responsible for overseeing a fifth of the U.S. economy. Many of President Trump’s early hires were mediocrities or worse. Mr. Gottlieb quickly proved himself an exception. He deserves credit for bucking the administration’s otherwise pervasive penchant for knee-jerk deregulation and reality denial.

In particular, Mr. Gottlieb advanced the federal government’s decades-long fight against tobacco with a vigor that one would not have expected from a Trump appointee. Congress in 2009 empowered the FDA to radically reshape the tobacco industry, and the agency made slow but steady progress during the Obama years. Mr. Gottlieb at first seemed to sympathize with business and anti-regulatory voices decrying federal rules on e-cigarettes and other products, delaying Obama-era rules on vaping products.

But he rolled out an ambitious plan to slash nicotine to minimally or nonaddictive levels in conventional cigarettes, the holy grail of anti-tobacco regulation that public-health advocates have been seeking for years. This could turn out to be one of the most important federal public-health interventions of our time, as smokers or would-be smokers instead use “products capable of delivering nicotine without having to set tobacco on fire,” as Mr. Gottlieb put it at the time.

Then, last year, Mr. Gottlieb’s FDA began a campaign against youth vaping, too, as teen e-cigarette use skyrocketed. Vowing that he would “not allow a generation of children to become addicted to nicotine through e-cigarettes,” Mr. Gottlieb announced a ban on convenience stores selling flavored vaping liquids that appeal to children, slapped stringent requirements on e-cigarette-makers and cracked down on unscrupulous retailers selling to minors.

Some anti-tobacco advocates wanted Mr. Gottlieb to move faster and further. But the FDA needed to balance two compelling interests: preventing teens from getting hooked on nicotine via vaping while keeping e-cigarettes available for adult smokers seeking alternatives to extremely deadly combustible tobacco products.

The risk now is that Mr. Gottlieb’s sense of purpose might depart from the FDA along with him. He is leaving before his legacy is secured and as Republican opposition to his regulatory moves is waxing. The agency must follow through on its plans to cut cigarette nicotine levels, and it must continue attacking youth e-cigarette use, lest federal inaction create a generation of nicotine addicts. Mr. Gottlieb’s successor also cannot ignore the opioid crisis, an issue on which Mr. Gottlieb has been outspoken but that still calls for more action.

The president has installed a number of highly qualified public-health officials. We hope he maintains the pattern by appointing a successor as serious and levelheaded as Mr. Gottlieb.