The Food and Drug Administration cracked down Wednesday on four manufacturers of caffeinated alcoholic drinks, giving them 15 days to stop adding caffeine to the products or stop selling them altogether.

Dr. Margaret Hamburg, the F.D.A. commissioner, said the drinks appeared to pose a serious public health threat because the caffeine masked the effects of the alcohol, leading to “a state of wide-awake drunk.” After a yearlong review found no conclusive evidence that the drinks were safe, she said, the F.D.A. decided the caffeine in them was an illegal additive.

In warning letters to the four companies — including Phusion Projects, which makes the top-selling caffeinated alcoholic drink, Four Loko — the F.D.A. said that drinking the beverages could lead to “hazardous and life-threatening situations.” The letters also warned that the F.D.A. could move more aggressively, seizing the beverages from store shelves and asking a judge to halt further sales, if the companies did not take corrective action.

Dr. Hamburg called the letters “a very important first step,” adding, “We hope we’ll be able to work effectively with the companies moving forward.”

The other manufacturers to receive warning letters were the Charge Beverages Corporation, New Century Brewing Company and United Brands Company Inc. Most of the products in question are carbonated malt beverages with fruit flavors and high levels of alcohol; Four Loko is 12 percent alcohol by volume and has up to 156 milligrams of caffeine per can, according to scientists who have analyzed it.

The Federal Trade Commission also took action against the four companies, warning that their marketing tactics might violate federal law and urging them to “take swift and appropriate steps to protect consumers.” This year, Senator Charles E. Schumer of New York asked the Federal Trade Commission to investigate whether the drinks, with colorful packaging and flavors like watermelon, blue raspberry and lemon-lime, were intended to attract under-age drinkers.

While the F.D.A. review dealt with more than two dozen makers of caffeinated alcoholic drinks, officials with the agency said they were taking action against only four for now because their products seemed to pose the biggest threat.

The popularity of the drinks has exploded over the last few months, and there have been numerous reports of young people falling ill after drinking them. Four Loko came under particular scrutiny after students who drank it this fall at Ramapo College in Mahwah, N.J., and Central Washington University in Ellensburg, Wash., ended up in emergency rooms, some with high levels of alcohol poisoning. The drink has also been blamed for several deaths in the past several months.

The F.D.A. announcement came a day after Phusion Projects, a five-year-old Chicago company, said it would stop putting caffeine in its drinks. The company’s founders said in a statement that they still believed it was safe to blend caffeine and alcohol, but wanted to cooperate with regulators.

Dr. Joshua Sharfstein, the F.D.A.’s principal deputy commissioner, said he viewed that announcement as a positive step but needed more details about how and when the company would reformulate the drinks.

Several states — including Michigan and Washington — banned the drinks on their own in recent weeks, and many more were considering similar action.

The F.D.A. began reviewing the drinks a year ago, at the urging of 18 attorneys general. At issue was whether adding caffeine to alcoholic beverages was “generally regarded as safe,” an agency designation that requires accepted scientific evidence.

The agency has never officially allowed manufacturers to add caffeine to alcohol. Under its regulations, soft drinks are the only beverages to which caffeine can be added, and only in concentrations of 200 parts per million or less.

Although there is little research on the effects of mixing caffeine and alcohol, several studies have suggested that people get more intoxicated and engage in riskier behavior when they drink the combination beverages than when they drink alcohol alone.

Ricardo Carvajal, a lawyer at Hyman, Phelps & McNamara in Washington and a former associate chief counsel at the F.D.A., said it was unusual for the F.D.A. and the F.T.C. to take joint action.

“The liability risk goes up considerably once you have not one, but two federal agencies either stating or suggesting there are violations of federal law at work,” Mr. Carvajal said.

The crackdown does not apply to caffeinated alcoholic drinks that do not come premixed, like a cocktail of Red Bull and vodka, but Mr. Carvajal said bars that served them could face heightened liability, too.

“If I’m a bartender and I have been serving essentially concocted versions of this,” he said, “am I going to think a little more carefully about that now?”