Pennsic, like most big, camping events, is a wet site, meaning that there will be drinking on site. While no alcohol can be legally sold on the campsite, there is ample opportunity to purchase, beer, wine or other drinks in Slippery Rock or the other little towns in the area. If you plan to drink at Pennsic, no problem (provided you are over the age of 21, the legal drinking age in PA). However, use good common sense and stay safe while doing it. Do not overindulge to a point where you become a hazard to yourself or others or spoil the property or fun of those attending the event.
For the non-drinkers, don't be surprised to see SCAdian folk drinking alcohol at the parties and activities around you. If you choose not to drink, that is also no problem: You will be in good company, as many folks at Pennsic do not drink. There is plenty to do at the War that does not require an alcoholic haze.
For those who are recovering from an alcohol problem, there are Alcoholics Anonymous meetings held at Pennsic. Locations and times are generally announced in the Pennsic program. The meetings take place, usually, in a private campsite and are organized by a private individual, not officially by the Pennsic staff.