I guess it depends on where you are in the process. If you have not submitted your SF-86, then put it down. You will get lots of scrutiny on it, but be open and forthright.
it should not be hard to find the CIAs drug policy. my guess is that 2 uses is not a DQ but you may have to wait a year or 2 to apply because they likely have policy of "0 times in the last X years......."
http://cia.gov/employment/faq.html#gr4
"What if I used drugs or still use drugs; will this disqualify me from employment?
To be considered suitable for Agency employment, applicants must generally not have used illegal drugs within the last twelve months. The issue of illegal drug use prior to twelve months ago is carefully evaluated during the medical and security processing. Drug abuse is one of the common reasons applicants are denied a security clearance. You cannot work for the CIA without a security clearance. You can find more information on this subject at Before You Apply on the How to Apply page. "
Don't know what to tell you. They are on a hiring binge right now, and if you are otherwise attractive to them, you might be able to finesse your way through. You may lose the time and energy, and emotionally, if you do not get it, but you will not get the job by NOT applying...
My opinion is that the majority of the folks in this country do not want anyone in the CIA who would lie to to get the job. You're probably screwed anyway because you have given enough information here for them to id you.