If you are taking a polygraph for employment as a law enforcement officer you should not be lying about anything.

Police work entails a lot of autonomy and trust, and lying to get what you want is not a trait that suggests you will be behave ethically in the future.

Take responsibility for your past actions and your choices.  If you behavior disqualifies you from this agency, apply elsewhere.  If your behavior was so irresponsible that you cannot get hired anywhere without lying then choose a different career.

You cannot behave unethically in order to get hired and then expect to behave ethically once you are on the job.

Oh boy, I find myself in agreement with Sarge. Yikes!!!!
LOL

So your telling us the only thing you smoked was marijuana? Ok, why on earth didn't you put that down on your appication packet? Atleast you could of asked your recruiter, or Human Resources. 
Chimps eh?
I'm thinking, or recommending you should look somewhere where chimps don't work at, your pleading to us about how your trustworthy got thrown out the window as soon as you admitted to lying. It doesn't matter now if you admit to it during your next test. 
The department doesn't have to prove anything your a candidate, not an employee. If they think something is fishy, and you lied... Need I say anymore? 
I'm still not for prescreen. But I think the examination honestly weeded out a liar... 

     This is an interesting thread.  I agree that law enforcement and government people need to be honest and truthful.  However, there is a difference between being honest and being a blabbermouth.  They really don't want to hear all of your dirty laundry.  I mean really, in order to even pass the test you have to lie.  They have to get you to lie about something and have you feel guilty about it to guage your lie reaction in the first place.  As long as it's not anything serious or problematic that's not what they're looking for.  In my 17 years of law enforcement experience, I have had to lie to do my job many times.  In fact you have to be good at lying.  I did some undercover work and that's all about being a good liar.   
     There are many investigative circumstances that require law 
enforcement personnel to lie.  You just can't lie in court or in a way that will violate federal/state law.  You can't lie about your casework etc. and have to be willing to tell the truth even when it's going to hurt you.  They same is true for other positions of trust.   
     My point is, going into the test planning to be completely honest may not be the best strategy.  I know I may not seem like the best person to get advice on how to pass, since I failed my last three polygraphs.  I did pass the pre-employment polygraph and a CI polygraph though, so I know a little about it.  In the pre-employment screening they do want to see if you can keep your mouth shut about stuff that is just embarrasing and doesn't matter and project the image of a LEO.  For the most part LEOs are not very forthcoming with personal nformation and that is the most appropirate demeaner to have.  Just don't lie about the important stuff.