Many people enjoy letting their hair down at the Christmas party, and may have a drink or two after a hard year's work, but staff shouldn't become so drunk that they cause harm to others (or themselves) at, or following, an employer's function.
Excessive drinking can cause staff to forget personal boundaries. It is in these types of situation that claims for sexual harassment, for example, can arise.
Employers are responsible if anything goes wrong at a work function, so where alcohol is available they should put contingency plans in place (for example, making sure everyone has a lift home or a designated driver, or there are taxis available).
If staff are expected to report for work the next day, they should take care not to drink too much, particularly if their work involves driving or operating machinery or other potentially dangerous equipment.
It may be sensible for your employer to introduce an alcohol policy, so that everyone knows where they stand.