Alcohol use in the workplace is obviously unsafe and potentially an OSHA violation. Whether it is technically illegal is irrelevant. Please do not drink on the job as it creates all types of exposure for your employer and increases your risk of injury.
Please check the law of your state
I interpret your question to be implying that others, not you, have been drinking. I agree with another responder that whether it is "illegal" is not the issue. If you know others are drinking, it's a danger to you, to them, and to all others in the workplace. You need to consider how and to whom to report this. For your own protection, you would not want it to be learned later, after an alcohol-related accident, that you knew of the drinking and didn't mention it to anyone.
One way to do this is to report it anonymously, but also put it in writing in a letter, mail the letter to yourself, and leave it sealed. That way, (1) it gets reported; (2) no one retaliates against you for reporting; and (3) if it ever comes up as a question of whether you knew of the drinking and did nothing, you have your letter to prove you reported it. But the letter stays sealed if the issue never comes up, and your anonymity remains protected.