It's not all government jobs, but typically high end military and law enforcement jobs will make that a requirement. I don't know exactly what their rational is; I've heard different things, but mostly I think they're looking for exemplary character, and breaking the law for recreational purposes in your spare time disqualifies you from having that, in their eyes.

For those types of jobs they do what is called a Single Scope Background Investigation, and there is about 10 pages out of a 100 page form that deal with drug use. You need to be honest, because they also ask you to put down about 20 names and numbers of family, friends, and co workers; and they will even go through those people to find other people you didn't put down and potentially coach, and ask them about it. They have professionals that do this for a living, and will purposefully trip people up that you've even coached well, so basically they find out. And I've seen it DQ some really top notch guys.

I knew a guy who applied to be a U.S Marshal, he was a D1 athlete, and captain of his time, did community service, went to a great school and had good grades. He got all the way up to the background portion, admitted to using marijuana several times in college as well as using MDMA at a few parties; nothing major. They ended the investigation right there and told him to leave.

I have heard that some drugs they give you a amount of time from, but it depends on the market. An FBI recruiter told me a story about a guy who took steroids in college, they told him if he came back in 10 years they would look at it again, as long as he passed a poly saying he hadn't used in those 10 years. He went and did another job for a whole decade, came back, and got a job with them.

They take it pretty seriously.