'I Was Drunk' Is Not a Defense to a Crime
There are many criminal defenses you can raise at trial, but being drunk is generally not one of them.
The law does differentiate between voluntary and involuntary intoxication. The involuntary type can often be a defense, but you have to prove that you didn't know you would become intoxicated -- for example, by showing that someone else spiked your drink. If you had a beer knowing it was beer, however, that's voluntary.
The weird part is that on the surface, voluntary intoxication seems like it should get you out of trouble. But courts generally don't allow it. Here's why:
Criminal Intent and Liability
Most crimes require some element of intent. It's not a crime if it happens by accident.
In some cases, a person will get so intoxicated that he arguably can't form the necessary intent to commit a crime. Perhaps he didn't intend to get into a fight or realize he would hit someone hard enough to kill, but it happened.
Some states flat-out prohibit the use of voluntary intoxication as a defense in those cases. States that do allow it generally use it to mitigate the charge, not to drop the charges altogether.
Intoxication and Intent
When people get drunk and commit crimes, courts generally just shift when the "intent" element of the crime occured. For example, courts have found that the intent to get voluntary intoxicated is enough to fulfill the intent element of most crimes.
The only real exception to this, where voluntary intoxication will act as a defense, is theft. If a person gets drunk and doesn't realize he's taking property that's not his, it can sometimes act as a defense to theft. But it's not a free pass, as those individuals can still be sued for civil damages.
Bottom line: Being drunk is often used as an excuse for what happened the night before. But if what happened is a crime, being drunk will not be a defense.