I have a clearance and a somewhat checkered past.

A lot of what they look for is consistency in accounts of previous behaviors between you and any references / interviews they undertake, trying to assess if you're currently honest.

Another big thing they are trying to decide is if you are blackmailable.

(throwaway because I have the sort of clearance you are not supposed to advertise that you have)

John McAfee discussing getting his security clearance when he worked for Lockheed-Martin: "They asked me very revealing questions.... Had I ever taken drugs? Yes. What kinds? Almost every kind. Uh, how much? A lot. Have you ever sold drugs? Yes. So I assumed I would never get the clearance but I did and it came in very quickly."

(clearly decades ago, and he's less than reliable, but fun nonetheless!)

It's not so much a problem if you do drugs.

It's a problem if you do drugs and are hiding it from someone (say, family). Then you can be blackmailed.

Can confirm (source: I have a friend that works for a three-letter agency). Before I sit for a clearance interview, I asked him about it. I used to think I'd probably be disqualified, but I learned that was not the case at all. Much of what they're trying to determine is your exposure to potential blackmail.

How recent? Posts like "FBI can't find hackers" have been frontpaging for years but people who have responded to the ads say they were dismissed at the clearance step or advised to not continue due to it.