Please keep in mind that field searching can produce odd results with variables. A keyword can appear in the question text:
Q: How often do you use heroin?
Or in the answer text:
Q: What illegal drugs do you use?
3) Crystal meth
Use quotes to indicate specific phrases; use the minus sign to remove certain types of results:
"income level" -conservative
A Boolean "and" is implied in the search; you can use a Boolean OR.
heroin OR marijuana
The search automatically does stemming (more info); there's no need to type in an asterisk. It's also case-insensitive.