The U.S. Food and Drug Administration has sent out warning letters to four makers of caffeinated alcoholic drinks to warn that the caffeine they directly include in their products is an "unsafe food additive."

The announcement Wednesday follows a year-long study by the agency into the safety of the drinks, which are available in about 40 varieties in the U.S.

The FDA warning effectively means a ban on the drinks in the U.S. The agency says it has given the drink makers 15 days to respond to the warning letters "and then may proceed to court to stop their sale."

Four states have already banned the drinks after a number of car crashes and personal accidents were linked to the beverages.

The agency noted in a statement that the combination of caffeine and alcohol in the products poses "a public health concern." They said the caffeine can "mask cues" that drinkers may use to determine how intoxicated they are.

"This means that individuals drinking these beverages may consume more alcohol -- and become more intoxicated -- than they realize," the FDA said.

"Studies suggest that drinking caffeine and alcohol together may lead to hazardous and life-threatening behaviors. For example, serious concerns are raised about whether the combination of alcohol and caffeine is associated with an increased risk of alcohol-related consequences, including alcohol poisoning, sexual assault, and riding with a driver who is under the influence of alcohol."

The letters were sent to the makers of such brands as Joose, Core, Moonshot and Four Loko.

The fruit-flavoured Four Loko is the top-selling alcoholic energy drink in the U.S. and has been nicknamed "blackout in a can." It contains the same amount of alcohol as two beers and about as much caffeine as a cup of coffee in each can.

The maker of that drink, Phusion Projects, unexpectedly announced late Tuesday that it would reformulate its drinks to remove the caffeine, guarana and taurine from its products.

In a statement, the company's three co-founders said they were making the move after unsuccessfully trying to deal with what they called "a difficult and politically-charged regulatory environment at both the state and federal levels."

The company insists its product is as safe as any other alcoholic beverage.

"...If our products were unsafe, we would not have expected the federal agency responsible for approving alcoholic beverage formulas – the Tobacco Tax and Trade Bureau (TTB) – to have approved them. Yet, all of our product formulas and packaging were reviewed and approved by the TTB before being offered to consumers."

In Canada, it is not permitted to directly add caffeine or vitamins to alcoholic drinks. The only foods that are allowed to contain added caffeine are carbonated soft drinks.

But alcoholic drinks that contain caffeine are allowed if that caffeine is derived from "natural" sources, usually guarana. But at the moment, there is no regulation on how much "naturally derived" caffeine an alcoholic drink can contain.

In a statement to CTV News, Health Canada noted that the types of drinks under scrutiny by the U.S. Food and Drug Administration would not be permitted for sale in Canada.

"Canada has far tighter control on the addition of caffeine to foods under the Food and Drug Regulations," the statement says.

"Health Canada continues to advise Canadian liquor boards that alcoholic beverages containing energy drinks or directly added caffeine are not allowed for sale in Canada and continues to work with the CFIA to ensure compliance with the Canadian regulations."

The agency also says it advises consumers not to mix energy drinks with alcohol.

Yet there appears to be no regulation of a popular drink sold in Canadian bars: a can of Red Bull energy drink served with a shot of vodka.

The energy drink issue in Canada could become clearer in the coming weeks. The federal health minister has hinted that's when recommendations from an expert Canadian panel could be made public.

Those recommendations may or may not include new rules on alcoholic energy drinks. But it is expected that there could be changes coming to the way regular energy drinks are labelled and displayed at convenience stores.