I observe that at the airports your luggage is scanned but the people get through just fine, and often carry all manner of parasites, viruses, and bacteria with them.

A sufficiently clever chemist could transform narcotics like cocaine and fentanyl into a stable substance that resists metabolism, then inject it into a carrier.

That carrier could then saunter across the border through any airport, and once on the other side could have the drugs extracted from their blood via some kind of clever filtration system and then reverted back to their original forms.

Of course, the amount that you could actually transport in this way is probably minuscule, but the idea is still fascinating. I've handwaved this, because it could in fact turn out to be an enormously complicated procedure indeed, far beyond the difficulty of simply synthesizing the drugs from scratch on the other side.