Washington (CNN) -- The U.S. Food and Drug Administration told the manufacturers of seven caffeinated alcoholic beverages Wednesday that their drinks are a "public health concern" and can't stay on the market in their current form.



The move follows a year-long review by the FDA, which gave the companies 15 days to either reformulate their products or face possible seizure under federal law, said Dr. Joshua Sharfstein, the agency's principal deputy commissioner. Experts have said the caffeine used in the beverages can mask the effects of alcohol, leaving drinkers unaware of how intoxicated they are.



"FDA does not find support for the claim that the addition of caffeine to these alcoholic beverages is 'generally recognized as safe,' which is the legal standard," Sharfstein told reporters. "To the contrary, there is evidence that the combinations of caffeine and alcohol in these products pose a public health concern."



One of the companies that received warning letters was Phusion Projects, of Chicago, Illinois, which makes Four Loko -- a drink nicknamed "blackout in a can" by some users. The company announced Tuesday that it was dropping caffeine and two other ingredients, guarana and taurine, from Four Loko ahead of the FDA's expected action.



The other companies are San Diego, California-based United Brands, which manufactures the Joose and Max brands; Portland, Oregon's Charge Beverages Corp., which sells Core High Gravity HG, Core High Gravity HG Orange, and Lemon Lime Core Spiked; and New Century Brewing of Boston, Massachusetts, which makes Moonshot.



There was no immediate response to the announcement from those firms. United Brands said Tuesday that it would review any new FDA guidelines, but noted it was unaware "of a single incident of injury or other harm associated with its products."



The FDA has no authority to ban the products outright. But Sharfstein said the review is still going on "and may lead to action against other, similar products." The states of New York, Washington, Utah, Michigan and Oklahoma already have taken steps to remove similar drinks from store shelves.



The FDA began its review in November 2009, after complaints from officials in several states. The controversy exploded in October, when nine underage students at Central Washington University were hospitalized after drinking Four Loko, both on its own and mixed with other drinks, police reported.



Critics say drinks like Four Loko mix as much as three cups of coffee with three cans of beer and are designed to appeal to younger consumers accustomed to consuming high-caffeine energy drinks. A 23.5-ounce can of Four Loko contains either 6 or 12 percent alcohol by volume, depending on state regulations.



Phusion has compared Four Loko to popular drinks like rum and cola or Irish coffee, but said it was changing its formula in the face of "a difficult and politically-charged regulatory environment."



"By taking this action today, we are again demonstrating leadership, cooperation and responsible corporate citizenship," the statement said.



Dr. Orly Avitzur, medical adviser to Consumer Reports magazine, said Phusion's announcement addresses "the worst of the problem," but leaves behind "a very high-alcohol beer."



"This is only a partial solution," Avitzur said.



Combining large amounts of caffeine and alcohol produces "a wide-awake drunk," said Dr. Mary Claire O'Brien, an associate professor of emergency medicine at Wake Forest University's medical school. But since caffeine's effect wears off faster than alcohol's, she said, drinkers are left with "much more alcohol than he would have been able to tolerate -- and that leads to blackouts."



"If you're going to drink caffeine, drink it responsibly. If your going to drink alcohol, drink it responsibly," said O'Brien, who has studied the drinks extensively. "But mixing them is dangerous, because you might not know when you've had too much to drink."