In the midst of his fight against teen vaping, which he attacked as “epidemic,” U.S. Food and Drug Administration (FDA) Commissioner Scott Gottlieb has resigned after less than two years at the helm.

In a letter, addressed to Secretary of Health and Human Services Alex M. Azar, he resigned, effective in one month.

Gottlieb leaves in the midst of the teen vaping battle and the opioid crises.

He noted what he considered to be a number of FDA achievements during his tenure, including new policies “to reduce the morbidity associated with tobacco use; to confront teen use of e-cigarettes; to decrease the rate of new opioid addiction; to improve access to affordable generic drugs; to modernize the development process for novel medical technologies like gene therapy and targeted medicines; to implement measures to improve food safety and our ability to identify and track outbreaks of foodborne illness; and to reduce the burden of chronic disease through better information and diets.”

He noted that “a record number of generic medicines, novel drugs, and novel devices” had been approved in 2017 – and that FDA beat that record in 2018. He stressed that FDA “set in motion a historic modernization” of both the Office of New Drugs and of the Office of the Commissioner.

In addition, the agency furthered “new approaches for the modern and efficient regulation of cell based regenerative medicine, complex generics, targeted cancer drugs, dietary supplements, digital health tools, and personal genetic tests,” he added.

Beyond that, new pathways for novel devices were created, including “modernizations of the 510(k) process,” he said. These would let device makers use the less demanding practice of comparing a new device to older ones on the market if they can show that it's “at least as safe and effective" as one that was already approved, according to numerous reports, including by HCB News in December, 2018.

"What we want to do is constantly push the market toward incorporating better technology and better capabilities by advancing the predicates and always looking forward so the ... predicates the device companies are using as the basis of their approvals are constantly incorporating newer and better technology to make the devices better and safer," Gottlieb said in an interview at the time with CNBC's Becky Quick.

He also highlighted the FDA's enforcement arm, which confronted “bad actors that put Americans at risk.” The agency “cracked down on bogus stem cell therapies, on sham homeopathy, on unsafe medical device products, on tobacco sales to minors, on unsafe dietary supplements, and on kratom.”

During his leadership, the agency also supported “major legislative efforts to secure a more modern framework for the efficient regulation of diagnostic tests and over-the-counter drugs” and made “efforts to address the opioid crisis with historic new authorities and resources,” as well as a focus on care for the troops and interdiction work inside the International Mail Facilities.

The FDA also had to face historic crises during Gottlieb's 23 months, including managing drug shortages in Puerto Rico, and protecting the public during the shutdown.

The move came as a surprise, considering that Gottlieb had tweeted January 3, that rumors he was quitting were untrue.

“I’ve heard from friends contacted by an online pharma news pub that’s preparing a story speculating that I’m leaving #FDA,” he tweeted, adding, “I want to be very clear – I’m not leaving. We’ve got a lot important policy we’ll advance this year. I look forward to sharing my 2019 strategic road map soon.”

Apparently, this has been in the works for a while. A source inside the FDA reportedly advised that it's happening because of “'the difficulty of commuting back and forth between' Washington and Connecticut, where Gottlieb's three young children live,” according to Axios.

"All of us at HHS are proud of the remarkable work Commissioner Gottlieb has done at the FDA," Azar said in a statement. "He has been an exemplary public health leader, aggressive advocate for American patients, and passionate promoter of innovation. I will personally miss working with Scott on the important goals we share, and I know that is true for so many other members of the HHS family," according to CBS.

But some consider Gottlieb's approach too permissive – speeding and easing regulations and approvals.

“We’re not sad to see him go,” director of the Health Research Group Michael Carome, a Gottlieb-appointment opponent, told the Los Angeles Times, adding, “he was entangled in an unprecedented web of ties to Big Pharma.”

His detractors point to the approval of baricitinib, sold as Eli Lilly's Olumiant, for rheumatoid arthritis – even though drug was rejected in 2017 because of reports of blood clots, according to the paper.

Critics also suggest that the agency “undermined” its own efforts to combat the opioid crisis, when it OK'd the potent new under-the-tongue opioid drug, Dsuvia, according to the paper.