President Trump has long railed against drug traffickers. He has said they should be given the death penalty “for their heinous acts.” On the first day of his second term, he signed an executive order listing cartels as “terrorist organizations.”

But many public health and addiction experts fear that his budget proposals and other actions effectively punish people who use drugs and struggle with addiction.

The Trump administration has vowed to reduce overdose deaths, one of the country’s deadliest public health crises, by emphasizing law enforcement, border patrols and tariffs against China and Mexico to keep out fentanyl and other dangerous drugs. But it is also seeking huge cuts to programs that reduce drug demand.

The budget it submitted to Congress this month seeks to eliminate more than a billion dollars for national and regional treatment and prevention services. The primary federal agency addressing drug use, the Substance Abuse and Mental Health Services Administration, has so far lost about half its workers to layoffs under the Trump administration and is slated to be collapsed into the new Administration for a Healthy America, whose purview will reach far beyond mental illness and drug use.

And if reductions to Medicaid being discussed by Republicans in Congress are realized, millions of Americans will be unable to continue, much less start treatment.

The White House did not respond to requests for comment. The budget itself says that ending drug trafficking “starts with secure borders and a commitment to law and order” and that it is cutting addiction services deemed duplicative or “too small to have a national impact.”

Those cuts are agonizing, public health experts say, because they come just as the country is making sustained progress in lowering the number of fentanyl deaths. Many interventions may be contributing to that progress, including greater availability of the overdose reversal spray naloxone; more treatment beds, sober housing and peer counseling; and declines in the strength and quantity of the illicit drug supply, they say. But studies so far have not demonstrated convincingly which of those factors merit greater focus and investment.

“It would be a tragedy if we defund these programs without fully understanding what’s working and then our overdose rate starts to climb again,” said Dr. Matthew Christiansen, an addiction medicine physician in Huntington, W.Va., a city once labeled ground zero for the opioid crisis.

A letter signed by more than 320 behavioral medicine academic experts, sent Monday to congressional leaders, decried the cuts, including those to “community-based naloxone distribution, peer outreach programs, drug-use-related infectious disease prevention programs and drug test strip programs.”

The president’s budget calls for ending grants for “harm reduction,” a strategy to prevent disease transmission and keep drug users alive that has become largely accepted by mainstream addiction treatment providers.

The budget derides federal financial support for “dangerous activities billed as ‘harm reduction,’ which included funding ‘safe smoking kits and supplies’ and ‘syringes’ for drug users.”

That language is a callback to false reports in 2022 that a $30 million federal harm reduction grant could be used to purchase pipes for smoking crack and meth. In fact, a small portion of that grant, designated for “safer smoking kits,” was for supplies like alcohol swabs and lip balm. The grant also supported programs in states that permit sterile syringe exchanges, effective in reducing hepatitis C and H.I.V. infection rates.

“You can’t just tell people to stop using drugs with a snap of the fingers,” said Dr. Christiansen, a former director of West Virginia’s drug control policy. “These are tools to reduce the harm of opioids while also helping them be successful long-term.”

According to the federal agency’s annual survey of substance use, in 2023, 27.2 million Americans ages 12 or older had a drug use disorder, 28.9 million had alcohol use disorder, and 7.5 million had both.

The budget does leave intact block grants for states to combat addiction and mental illness. But without the agency’s additional grants, hands-on training and monitoring, in addition to possible Medicaid reductions, states will not be able to afford the many medical and social services required to prevent and treat addiction, Dr. Christiansen said.

David Herzberg, a professor of drug policy and history at the University at Buffalo, said that Mr. Trump’s almost single-minded linking of the nation’s drug problems with border issues harks back to late 19th-century America, when the government associated opium dens with Chinese immigrants. Fearing the incursion of Chinese workers and inflamed by press reports of Chinese men using opium to lure young white women into prostitution, Congress severely restricted Chinese immigration.

Then as now, Mr. Herzberg said, political conservatives found that targeting foreign drug suppliers was a muscular means of advancing broader agendas.

In contrast with highly publicized drug seizures, people who chronically use drugs have become afterthoughts, usually visible only as street irritants, their addiction perceived to be the result of their own choices, he said. Elected leaders who advocate for their welfare risk being tarred as soft on crime.

“If politicians are going to stick their necks out for them, I would be shocked,” Mr. Herzberg said.