I recently got this question when returning from Amsterdam.

I truthfully answered that I had not used any marijuana. "Not even a little bit?" No, not even a little bit.

But I'm wondering... if I had used marijuana in Amsterdam, and truthfully told Customs about it, what could they / would they do? There's no law against Americans using marijuana while outside the US, so it seems like maybe a pretext for a search, but would they try to charge me with something?