The flow of illegal drugs into the country is a major concern not only for law enforcement but also for public health officials. In studying this, one should separate out marijuana from other drugs.
Marijuana poses a somewhat unique set of issues for two reasons. The first is that it is legal in several states for recreational use and also has legitimate medical uses for which it is legal in other states. Marijuana is grown in the United States as well as imported. Since it is far bulkier than other drugs, it is actually much harder to import as, for example, $10,000 worth of marijuana weighs far more that $10,000 worth of heroin.
As marijuana becomes more widely legalized and grown commercially, illegal importation will naturally decline and become unprofitable. Another way to end illegal important of marijuana would be to treat it like alcohol and tobacco, allowing legal but controlled importation, with controls for quality and appropriate taxation. This would end profits and incentives for much illegal importation.
Other drugs, especially heroin and its relatives, are overwhelmingly imported through either mail or ports of entry. Careful monitoring of packages entering the country, including requiring manifests and increased checking for the presence of drugs, would help—especially in the case of fentanyl, a substance for which a very small quantity can be extremely profitable and which therefore is particularly suited to smuggling via mail.
For drugs that enter via ports of entry, staffing levels and training of customs officials are extremely important, as is continuing to develop drug sensing technology. For drugs entering between ports of entry, tunnels are a known problem. Drone surveillance and other technological solutions are likely to be useful in prevention.
Eventually, though, the main cause of drug smuggling is demand. Funding programs to treat addiction and its causes addresses the root of the problem rather than just the symptom.