DoD Security Clearance Question?

This is a cross-post from the Military section, as this is not specifically a Military job. It will be a civil job with the government.

I am in a position in which, for the program I'm currently in, I will need to get a US government security job. This obviously means getting a security clearance. I do not anticipate needing a TS for the first job due to time constraints, but it is still a possibility.

I am a little concerned because, despite a squeaky clean history (with the exception of occasional internet piracy which I have stopped), I have used drugs one time in the past. 3 years ago I did LSD once, and have since then refused any further drug use offers with no exceptions. I was a college sophomore at the time.

Now of course I am putting this on the SF86, but does anyone know what possibility this has of being a DQ for a clearance? I am a little bit nervous, because due to the requirement to obtain a government job, a rejection could be problematic.

Thanks for any input.