Any drug use is a big deal. I told the recruiter about the drug use, and then he starts telling me I'll have to get a top score on the PFT just to get in at all. He said this qualifies as a district level waiver, which will disqualify me from a lot more jobs than the waiver I'd have to get for age or other lesser issues. There's an Army recruiter right next door, so I asked the Marine recruiter "Well what does the Army think about it?" He walked to the other office and back and told me it would be a disqualification for Army enlistment. He said anything with the name of that substance is a no go for them.
So, yeah, it is a very big deal. Any previous use of a narcotic, as little as one use, even if it was many years ago, can prevent enlistment.