Caffeinated alcoholic beverages such as Four Loko and Joose are being removed from store shelves amidst concerns over their powerful stimulant-sedative combination. Now the federal government is weighing in on these party drinks, saying they are unsafe and should not be marketed to young people. The U.S. Food and Drug Administration (FDA) on Wednesday sent lengthy warning letters to four makers of such caffeinated alcoholic beverages—Charge Beverages Corp., New Century Brewing Co., Phusion Projects and United Brands Co.—informing these companies that their products are being marketed in violation of the Federal Food, Drug, and Cosmetic Act.

After a yearlong investigation of alcoholic drinks with caffeine added, the agency has determined these beverages are a "public health concern" and has warned the four companies that the caffeine added to their malt alcoholic beverages, including Four Loko, Joose and Core, is an "unsafe food additive." These companies now have 15 days to explain to the FDA how they will address this problem.

Phusion Projects, which makes Four Loko, has attempted to preempt the government's crackdown, announcing Tuesday that it will remove caffeine, guarana and taurine from its products, even though the company contends that the combination of alcohol and caffeine is safe. In a statement on the company's site, company co-founders Chris Hunter, Jeff Wright and Jaisen Freeman note that all of Phusion's product formulas and packaging were reviewed and approved by the U.S. Department of Treasury's Tobacco Tax and Trade Bureau before being offered to consumers.

In a press release issued prior to the FDA's announcement, United Brands CEO and president Michael Michail stated that his company would adhere to federal and state health and safety guidelines. Charge Beverages and New Century had not issued statements related to the FDA's announcement as of Wednesday afternoon.

At a press conference announcing the FDA's findings, Joshua Sharfstein, FDA principal deputy commissioner, said he wouldn't characterize the agency's action as "an outright ban. Our conclusion is that the drinks are adulterated and do not meet safety standards."

The warning letters read like legal documents and explain the FDA's position that alcoholic drinks with added caffeine are in violation of its regulations. If the manufacturers do not respond in the allotted two weeks, the government could seize their products or file an injunction to prevent them from continuing to make these beverages.

The FDA says it examined the published peer-reviewed literature on the co-consumption of caffeine and alcohol, consulted with experts in the fields of toxicology, neuropharmacology, emergency medicine and epidemiology as well as reviewed information provided by product manufacturers. FDA says it also performed its own independent laboratory analysis of these products and listened to experts who have raised concerns that caffeine can mask some of the sensory cues individuals might normally rely on to determine their level of intoxication.

A typical alcoholic energy drink is 24 ounces (0.7 liters) and has a 12 percent alcohol content—compared with a 12-ounce (0.35-liter) can of beer, which normally has 4 to 5 percent—plus the caffeine equivalent of five cups of coffee. "The consumption of these drinks may lead to serious problems," Thomas Gould, an associate professor of psychology at Temple University in Philadelphia, recently told Scientific American. "If co-administration of alcohol and caffeine allow people to drink for longer periods of time because of reduced sedation, they may consume more alcohol; this could lead to serious health risks, such as near-lethal blood alcohol levels as seen in the Central [Washington] University case." Last month nine C.W.U. students were hospitalized after drinking caffeinated malt liquor.

Charge Beverages, New Century Brewing, Phusion Projects and United Brands were singled out based on three criteria: the companies advertise the direct addition of caffeine to their products, they sell their products in single serving containers and they are easy to obtain and test.

Caffeinated alcoholic beverage makers who did not make the FDA's list are not off the hook, Sharfstein warned, adding, "We expect these letters to be read across the industry."

The FDA's actions come after several states, including Michigan, New York and North Carolina, have called for caffeinated alcoholic beverages to be removed from store shelves.

When asked whether the FDA was concerned that there would be run on these products by thirsty young drinkers, Sharfstein said, "We don't think it's a good idea for people to rush out and buy these products."