Any violation of a controlled substance law, anywhere in the world, incurs a lifetime inadmissibility to the U.S. If you're citationophilic, it's section 212(a)(2)(A)(i)(II) of the Immigration and Nationality Act or 8 USC 1182(2)(A)(i)(II). It does require a *violation* of the law, so merely having used drugs 20 years ago, five years ago, or five minutes ago does not necessarily incur the ineligibility. If CBP already knows from past encounters (or a variety of other sources) that you've been convicted of a drug offense, though, then if you just show up at the border, you are pretty much toast. There are a few arcane exceptions to that rule (almost all of which require the offense to have been committed before age 18), but broadly speaking anyone with a drug violation on their record at any point in the past will have a much harder time traveling than they would otherwise--visa-free travel is right out. That said, the ineligibility is pretty routinely waived in the course of a visa application if the traveler is otherwise an upstanding citizen, honest about their history, and not likely to re-offend--so, as a practical matter, someone who smoked a J or several in college will have to jump through a few more hoops for having done so, but will probably still be allowed entry in the end, all else being equal.

Actively LYING about this sort of thing to a CBP or visa officer will quite often incur a permanent ineligibility under still another law section, and this may well torpedo the traveler's credibility beyond repair. So, don't do that.

Holy sweet muppety Zeus, what terrible advice. The answer to ANY question put to you by a federal officer, if you don't like where the conversation is headed, is the truth or silence. You do not know what information CBP is working off of, and material misrepresentation is a much more durable form of forever ban than a minor, ancient-history drug offense is. Literally being caught with a joint in your pocket at the inspection is less likely to result in a permanent ban.