 It declared that beverages that combine caffeine with alcohol, such as Four Loko energy drinks, are a "public health concern" and couldn't stay on the market in their current form.The FDA also stated that concerns have been raised that caffeine can mask some of the sensory cues individuals might normally rely on to determine their level of intoxication.Warning letters were issued to each of the four companies requiring them to provide to the FDA in writing within 15 days of the specific steps the firms will be taking.