When you are with people who are doing something, even when you arent doing it, you feel the same effect. It works depending on if you mentally believe in it or not.
Since I'm straight edge, when my friends drink, i use the placebo effect and feel the same as they do.
This is mind over matter; when somebody believes that something will happen to his health, it does even when it wouldn't have otherwise. This generally happens when doctors tell patients that their health will improve even though there's no logical reason to believe that; the patients' optimism then helps heal them.
The doctor gave Jimmy sugar capsules and told them it would cure his headaches; it did even though the capsules didn't do anything. Classic placebo effect!