Here's the scoop...
First, stay the h*ll away from your nieghbor.
Second, be VERY prepared to discuss, at length, your association with ANYONE who has done drugs in your presence. You'll likely be asked about it in a background packet. Don't lie. DO NOT LIE! Catch my drift?
It's better to explain to the investigator the truth about any associations, experiences, or knowledge you or someone you know of, that has done, or continues to use.
Think of it this way, it'd be better to be let go because you told the truth, than to be two weeks into an academy and be fired because they decided to go knocking on your neighbors doors asking about what kind of neighbor you are. If you're fired from a fire job, it's going to be even more difficult to get a job w/ another agency.
"Oh, yeah, Joe...(cough, cough bong in hand)we watch Days of Our Lives together every day". Not the people you want your background investigator finding out about, but be prepared to discuss it anyhow.
Best of luck. Any other questions, feel free to ask.