One can drink expired beer without fear of health problems. However, beer is not as flavorful beyond the expiration date. Most beer is not meant to age, so there is no benefit to drinking it beyond its expiration or "best before" date.
Hop-forward styles such as India pale ales tend to lose their flavors the fastest, after about 3 to 6 months, since the compounds responsible for hop flavor and aroma are very volatile. Beer that is exposed to excessive light for a long period of time can develop "skunky" flavors. There are some exceptions to the rule that beers do not improve with age, however; high-gravity, dark beers, such as barley wines and some stouts and porters tend to improve with time at cellar temperatures.
