From DEA’s December 2016 National Drug Threat Assessment. From GAO’s April 2018 report on synthetic opioids.

Here are similar photos from U.S. government reports, the left from the U.S. Drug Enforcement Administration (DEA) in 2016 and the right from the U.S. Government Accountability Office (GAO) in 2018. They show estimates of a lethal dose of fentanyl, a synthetic opioid that contributed to more than 19,000 U.S. overdose deaths in 2016.

The 2018 photo appears to show even less of the drug (2 milligrams), I don’t know whether that was deliberate. But the point is, it’s incredibly potent: 50 times stronger than heroin. First responders saving victims, and law enforcement agents and their dogs seizing smuggled fentanyl, must wear protective gear to keep from touching it.

It’s mostly produced in China, but Mexico is a major trafficking corridor, as GAO explains.

Mexican drug trafficking organizations act as the country’s primary conduit for Chinese fentanyl destined for the United States, purchasing bulk shipments and trafficking it—either alone or mixed with other drugs like heroin—across the U.S. border along established drug routes.… The illicit nature of these smuggling operations makes it difficult to quantify the volume of fentanyl flowing from Mexico to the United States. U.S. law enforcement agencies suggest that fentanyl may also be produced in Mexico with precursor chemicals sourced from China.

Like all other drugs smuggled across the U.S.-Mexico border except for marijuana (which is declining), fentanyl is overwhelmingly taken via the ports of entry: the legal openings in the border through which vehicles, cargo, and pedestrians pass under the scrutiny of U.S. Customs and Border Protection (CBP). The drug is small in volume and crosses hidden in vehicles or carried by pedestrians. Building a border wall would make no difference in fentanyl trafficking patterns.