A career as an FBI special agent means you'll be held to high legal and moral standards, but the bureau doesn't prohibit its agents from living normal, sociable lives. All federal government agencies have strict drug-free workplace policies and the FBI is no different. But the policy against drug use doesn't address alcohol use. That said, the FBI probably won't tolerate excessive drinking within its ranks because of the problems that can arise.
Sound Judgment and Discretion Required
FBI agents who imbibe probably don't have to worry about losing their jobs unless their alcohol use results in circumstances that harm themselves and others. Alcohol can affect anyone's judgment and decision-making capabilities, so it stands to reason that an agent should not engage in activities that could put her in a compromising position, whether she's on- or off-duty.