In general, the Pagan population tends to have a very liberal attitude about the reasonable use of alcohol. It's not uncommon to have wine at a ceremony, although there are a number of covens dedicated to serving people in recovery, and those groups naturally have alcohol-free rituals. Most Wiccans and Pagans will tell you that as long as you can maintain responsible behavior, the use of alcohol is a matter of personal choice.
It's nearly univerally agreed upon, however, that abuse of or dependence upon alcohol is something not to be looked at favorably. That's not to say that a Pagan gathering won't have some late-night mead-fueled revelry -- but consumption to the point of losing control is nearly always seen negatively. For one thing, it takes you out of control of your own actions. For another, it can put the well-being of others at risk.
As to the use of illegal drugs, while there are certainly people who indulge in them, no reputable coven will endorse the use of drugs in a ritual or ceremony (one notable exception to this would be the case of Native American rituals involving peyote). In fact, drug use is one of the big red flags to look for when seeking out a coven to join -- if someone tells you that getting baked is part of "honoring the Goddess", head for the door.
Pagans are big on the concept of personal responsibility -- and that means that if you choose to engage in negative, illegal, or harmful behavior, you need to be ready to accept the consequences of your actions.