The United States sustains a multi-billion dollar illegal drug market. An estimated 28.6 million Americans, or 10.6% of the population age 12 or older, had used illicit drugs at least once in the past month in 2016. The 2018 National Drug Threat Assessment indicates that Mexican transnational criminal organizations (TCOs) continue to dominate the U.S. drug market. They "remain the greatest criminal drug threat to the United States; no other group is currently positioned to challenge them." The Drug Enforcement Administration (DEA) indicates that these TCOs maintain and expand their influence by controlling lucrative smuggling corridors along the Southwest border and by engaging in business alliances with other criminal networks, transnational gangs, and U.S.-based gangs.

TCOs either transport or produce and transport illicit drugs north across the U.S.-Mexico border. Traffickers move drugs through ports of entry, concealing them in passenger vehicles or comingling them with licit goods on tractor trailers. Traffickers also rely on cross-border subterranean tunnels and ultralight aircraft to smuggle drugs, as well as other transit methods such as cargo trains, passenger busses, maritime vessels, or backpackers/"mules". While drugs are the primary goods trafficked by TCOs, they also generate income from other illegal activities such as the smuggling of humans and weapons, counterfeiting and piracy, kidnapping for ransom, and extortion.

After being smuggled across the border, the drugs are distributed and sold within the United States. The illicit proceeds may then be laundered or smuggled as bulk cash back across the border. While the amount of bulk cash seized has declined over the past decade, it remains a preferred method of moving illicit proceeds—along with money or value transfer systems and trade-based money laundering. More recently, traffickers have relied on virtual currencies like Bitcoin to move money more securely.

To facilitate the distribution and local sale of drugs in the United States, Mexican drug traffickers have sometimes formed relationships with U.S. gangs. Trafficking and distribution of illicit drugs is a primary source of revenue for these U.S.-based gangs and is among the most common of their criminal activities. Gangs may work with a variety of drug trafficking organizations, and are often involved in selling multiple types of drugs.

Current domestic drug threats, fueled in part by Mexican traffickers, include opioids such as heroin, fentanyl, and diverted or counterfeit controlled prescription drugs; marijuana; methamphetamine; cocaine; and synthetic psychoactive drugs. While marijuana remains the most commonly used illicit drug, officials are increasingly concerned about the U.S. opioid epidemic. As part of this, the most recent data show an elevated level of heroin use in the United States, including elevated overdose deaths linked to heroin and other opioids, and there has been a simultaneous increase in its availability, fueled by a number of factors including increased production and trafficking of heroin by Mexican criminal networks. Increases in Mexican heroin production and its availability in the United States have been coupled with increased heroin seizures at the Southwest border. According to the DEA, the amount of heroin seized in the United States, including at the Southwest border, has generally increased over the past decade; nationwide heroin seizures reached 7,979 kg in 2017, with 3,090 kg (39%) seized at the Southwest border, up from about 2,000 kg seized at the Southwest border a decade earlier.

In addition to heroin, officials have become increasingly concerned with the trafficking of fentanyl, particularly nonpharmaceutical, illicit fentanyl. Fentanyl can be mixed with heroin and/or other drugs, sometimes without the consumer's knowledge, and has been involved in an increasing number of opioid overdoses. Nonpharmaceutical fentanyl found in the United States is manufactured in China and Mexico. It is trafficked into the United States across the Southwest border or delivered through mail couriers directly from China, or from China through Canada.

Federal law enforcement has a number of enforcement initiatives aimed at countering drug trafficking, both generally and at the Southwest border. For example, the Organized Crime Drug Enforcement Task Force (OCDETF) program targets major drug trafficking and money laundering organizations, with the intent to disrupt and dismantle them. The OCDETFs target organizations that have been identified on the Consolidated Priority Organization Targets (CPOT) List, the "most wanted" list of drug trafficking and money laundering organizations. In addition, the High Intensity Drug Trafficking Areas (HIDTA) program provides financial assistance to federal, state, local, and tribal law enforcement agencies operating in regions of the United States that have been deemed critical drug trafficking areas. There are 29 designated HIDTAs throughout the United States and its territories, including a Southwest border HIDTA that is a partnership of the New Mexico, West Texas, South Texas, Arizona, and San Diego-Imperial HIDTAs.

Several existing strategies may also be leveraged to counter Southwest border drug trafficking. For instance, the National Southwest Border Counternarcotics Strategy (NSBCS), first launched in 2009, outlines domestic and transnational efforts to reduce the flow of illegal drugs, money, and contraband across the Southwest border. In addition, the 2011 Strategy to Combat Transnational Organized Crime provided the federal government's first broad conceptualization of transnational organized crime, highlighting it as a national security concern and outlining threats posed by TCOs—one being the expansion of drug trafficking.

The 116th Congress may consider a number of options in attempting to reduce drug trafficking from Mexico to the United States. For instance, Congress may question whether the Trump Administration will continue or alter priorities set forth by existing strategies. Policymakers may also be interested in examining various federal drug control agencies' roles in reducing Southwest border trafficking. This could involve oversight of federal law enforcement and initiatives such as the OCDETF program, as well as the Office of National Drug Control Policy (ONDCP) and its role in establishing a National Drug Control Strategy and Budget, among other efforts.