Admitting to having used LSD four decades ago is apparently enough to warrant a lifetime ban from entering the US:

Before long the customs guard was engrossed in an article Feldmar had published in the spring 2001 issue of the journal Janus Head. The article concerned an acid trip Feldmar had taken in London, Ontario, and another in London, England, almost forty years ago. It also alluded to the fact that he had used hallucinogenics as a "path" to understanding self and that in certain cases, he reflected, it could "be preferable to psychiatry." Everything seemed to collapse around him, as a quiet day crossing the border began to turn into a nightmare.

To my future background checkers, I would like to note that I have never commited any acts of moral turpitude, including but not limited to LSD use.