FDA Commissioner Margaret Hamburg says the mix of caffeine and alcohol can lead to what she calls "a state of wide-awake drunk." She says the evidence shows that the drinks have led to alcohol poisoning, car accidents and assaults.
Experts think the caffeine could mask a feeling of intoxication, leading to risky behavior.
If the companies keep making and marketing the drinks, which have been popular with college students, the government could seize the products.
The companies have 15 days to respond to the letters, either explaining how they will take the drinks off the market, or defending them as safe.
The maker of one of the drinks, Four Loko, announced Tuesday that it will remove the caffeine. The company's founders say they still believe a mix of alcohol and caffeine is safe, but that they're up against a "politically-charged regulatory environment."