Generally, for something like pot, agencies are going to want a year of abstinence before they'll talk to you. Likely, it'll come up in your clearance interviews and you may have to sign a piece of paper stating that you won't do it again. For longer term drug use or other drugs, the agencies may funny about it. If you're curious the Adjudicative Desk Reference (ADR) gives some guidelines on how the process works:

http://www.dhra.mil/perserec/adr/ADR_Version_4.pdf

As far as foreign national contact, most of the time, the feds just defer to the wording on standard form 86 (SF-86):

http://www.opm.gov/Forms/pdf_fill/sf86.pdf

It's long, but gives a pretty clear indication on the kinds of questions asked. Basically, they're looking for foreign contacts for which you've had "close and/or continuing contact" whose nature falls under 4 categories

1. Affection 2. Obligation 3. Influence 4. Common interest

If I recall correctly, that fourth one was added about five years ago. Anyway, check page 59 in the link.

You failed the interview for not pulling a James Clapper and lying.