You're out drinking, and you know you've had too much to legally drive. You don't want to pay for a cab home, though, so you decide you'll just swing by the local all-night coffee shop, drink a cup of hot brewed coffee, and then get in your car. After all, you've heard that it helps you sober up faster than just waiting for the alcohol to leave your system.
As prevalent as this belief is, it's a myth. Coffee won't make you more sober. It may make you feel more awake, due to the caffeine, but the amount of alcohol in your system won't change. If your Blood Alcohol Concentration was at 0.10 without coffee, it'll be at 0.10 with coffee.
Experts note that the only thing that really sobers you up is time. The alcohol has to metabolize and leave your system. This rate is around 0.015 per hour, when looking at BAC. That's true if you drink coffee or if you don't. Other common suggestions -- like taking a cold shower -- also don't help. They just wake you up.
Now, drinking the coffee could indirectly help. If your BAC is right at 0.08 and it then takes you an hour to walk to the coffee shop, order your coffee, get it, and sit at the table drinking it, your BAC could be down all the way to 0.65. However, the reason is that you waited an hour to drive, not that you drank the coffee.
If you are arrested on suspicion of drunk driving, especially if you really believed your BAC was in the legal range, you need to know what rights you have.
Source: Home Health Testing, "How Long Does Alcohol Stay In Your System?," accessed March 23, 2017