Citing the growing popularity of e-cigarettes, the Food and Drug Administration is considering adding warning labels on liquid nicotine used in so-called electronic "vaping" devices and is also mulling a requirement for the liquid to be sold in child-resistant packaging.

"The continuing rise in popularity of electronic nicotine devices (ENDS), such as e-cigarettes, which often use liquid nicotine and nicotine-containing e-liquids, has coincided with an increase in calls to poison control centers and visits to emergency rooms related to liquid nicotine poisoning and other nicotine exposure risks," the agency announced Tuesday.

The agency published its formal proposal in the Federal Register on Wednesday and provided the public 60 days to comment.

But it might be some time for regulations to actually take place.

That's because in April 2014, the FDA proposed regulations preventing the nationwide sale of e-cigarettes and nicotine liquids to minors and adding warning labels to them regarding health risks associated with nicotine. But no rules—which eventually could lead to a restriction of television advertising—have yet been announced.

"This proposed rule is the latest step in our efforts to make the next generation tobacco-free," then-Health and Human Services Secretary Kathleen Sebelius said at the time.

The FDA is taking so long because it said it is reviewing about 135,000 comments it received about the proposal.

Prominent medical groups wrote to President Barack Obama three months ago, urging the government to get going on the new rules, which also regulate cigars.

"It's no wonder use of e-cigarettes by youth has skyrocketed. This process has already taken far too long. We cannot afford more delays that allow tobacco companies to target our kids with a new generation of tobacco products," stated a letter (PDF) from nearly three dozen medical and health groups, including the American Heart Association and the American Academy of Pediatrics.

An April study from the US Centers for Disease Control showed that e-cigarette use by middle school and high school students tripled in use in one year. Among those in high school, the number jumped from 660,000 students to two million.

“We want parents to know that nicotine is dangerous for kids at any age, whether it’s an e-cigarette, hookah, cigarette or cigar,” CDC Director Tom Frieden said when announcing the report. "Adolescence is a critical time for brain development. Nicotine exposure at a young age may cause lasting harm to brain development, promote addiction, and lead to sustained tobacco use."

The states, however, are taking action. At least 44 of them bar the sale to minors of e-cigarettes and liquid tobacco.

The FDA regulates smokeless tobacco, traditional cigarettes and roll-your-own tobacco.