I guess it could depend on the position you are seeking and the agency,  but...

I haven't really looked into federal agencies but on the county,  state,  and local level, I've yet to run into an agency that cares about undetected underage drinking. So you should be ok there, but I wouldn't frame it as "getting drunk twice a month". The marijuana probably won't trip you up either.  You were young and never made a habit of it. 

The prescription drugs would be your biggest obstacle.  It may or may not be a DQ. I would think twice about disclosing that. But that's me. Maybe you can email the agency anonymously and ask about the prescription drug use. 

Timmyturner wrote on May 17^th, 2016 at 4:51pm:

I would concur with the previous response - it all depends on the specific agency involved.  Also, you didn't specify whether the agency was federal, state, or local.  In the case of the CIA, NSA, or FBI, I would say "SOL"!

They'll almost certainly ask you about your entire drug use history, including use of prescription drugs that weren't prescribed to you.

Many times, there are two standards of illegal drug use: marijuana and everything else. So the hydro use will be looked at with a discriminating eye.  How long ago was the use and what was your age at the time?  These factors may also be taken into consideration. 

Should you disclose it? It's your call. It doesn't look good,  that's for sure.  If you get the picture that it's an auto DQ, or something that will seriously damage your chances, I wouldn't disclose and I'd take my chances against the poly.  One thing that is an absolute in my book: stick to your story throughout the process.  If you don't disclose it on the application, don't change your story in the face of the pre poly process.  Typically local agencies and ones with more serious manpower issues will be more likely to overlook an issue and evaluate you as a candidate as a whole.