Every day, Dora Dantzler-Wright and her colleagues distribute overdose reversal drugs on the streets of Chicago. They hold training sessions on using them and help people in recovery from drug and alcohol addiction return to their jobs and families.

They work closely with the federal government through an agency that monitors their productivity, connects them with other like-minded groups and dispenses critical funds that keep their work going.

But over the last few weeks, Ms. Wright’s phone calls and emails to Washington have gone unanswered. Federal advisers from the agency’s local office — who supervise her group, the Chicago Recovering Communities Coalition, as well as addiction programs throughout six Midwestern states and 34 tribes — are gone.

“We just continue to do the work without any updates from the feds at all,” Ms. Wright said. “But we’re lost.”

By the end of this week, the staff of the agency, the Substance Abuse and Mental Health Services Administration, could be cut by 50 percent, according to senior staff members at the agency and congressional aides who attended briefings by Trump officials.

With just under 900 employees and a budget of $7.2 billion for large state grants and individual nonprofits that address addiction and mental illness, SAMHSA (pronounced SAM-sah) is relatively small. But it addresses two of the nation’s most urgent health problems and has generally had bipartisan support.

The agency’s broad mandate includes overseeing 988, the National Suicide and Crisis Lifeline, which fields millions of calls through state offices; regulating outpatient clinics that dispense opioid treatment drugs such as methadone; directing funds to drug courts (also called “treatment courts”); and producing nationwide annual surveys of substance use and mental health issues.

It provides best-practice training and resources for hundreds of nonprofits and state agencies, and helps establish centers that provide opioid addiction prevention, treatment and social services. It is also a federal watchdog that closely monitors the spending of taxpayer-funded grants for mental health and addiction.

Both President Trump and Robert F. Kennedy Jr., the federal health secretary, whose portfolio includes SAMHSA, have been outspoken about addressing the country’s drug crises. Mr. Trump has invoked overdose fatalities as a rationale for imposing tariffs on Canada, Mexico and China. Mr. Kennedy has often discussed his ongoing recovery from heroin addiction. During his presidential campaign, he produced a documentary about the impact of addiction in the United States that also explored different treatment options.

While the rates of U.S. overdose fatalities remain high, they have been declining consistently since 2023. Many drug policy experts say SAMHSA is the federal agency most directly responsible.

“Cutting SAMHSA employees without understanding the impact is extremely dangerous, given the behavioral health crises impacting every corner of our nation,” Representatives Paul D. Tonko of New York and Andrea Salinas of Oregon wrote in a letter to Mr. Kennedy, signed by 57 Democratic House members.

Reductions in staff, they argued, could lead to a surge in relapse rates, a strain on the health care system and poorer health outcomes overall.

Asked about the pending cuts, a spokeswoman for SAMHSA replied: “The important collaboration facilitated by SAMHSA’s regional offices continues, regardless of personnel changes, and SAMHSA staff remain diligently responsive to partners around the nation.”

On Tuesday, the Department of Health and Human Services announced that it was reducing its number of regional offices, which house agencies that include SAMHSA, from 10 to four.

Proposals to shrink staff sizes across government departments are due Thursday. In the last month, SAMHSA’s staff was reduced by roughly 10 percent through layoffs of workers in their probationary period, a designation that included people recently promoted to new positions. Last weekend, the agency’s employees and other personnel overseen by Mr. Kennedy received emails offering $25,000 to those who left their jobs by this Friday, characterized as a “voluntary separation.”

In interviews, a dozen current and former SAMHSA employees, including executives, said the threat posed by layoffs and policy shifts is beginning to be felt at sites everywhere, from the heart of troubled city neighborhoods to rural outposts. Some newer SAMHSA projects scarcely underway are in jeopardy, like one to map Chicago housing projects to better distribute the lifesaving overdose medication naloxone, and others to establish systems to speedily relay suicide intervention calls to on-the-ground response teams.

They said it was unlikely that funding for centers focused on treating the mental health or substance use disorders of specific populations, such as Black and L.G.B.T.Q. communities, would be reauthorized.

Regina LaBelle, the former acting director of the Office of National Drug Control Policy during the Biden administration, called the staff cuts “ shortsighted.”

“It might reduce numbers, but it also reduces oversight and accountability,” she said, by hindering the agency’s ability to monitor grant funds and collect behavioral health data.

During the Biden administration, the agency’s budget and staff grew substantially, a development that mental health and addiction experts described as an attempt to make up for persistent underfunding. In 2019, just before the onset of the pandemic, SAMHSA had about 490 full-time staff members and a budget of roughly $5.5 billion. According to the Centers for Disease Control and Prevention, there were 70,630 overdose deaths that year.

In March 2020, the pandemic bore down. Over the next three years, annual overdose fatalities soared to well over 100,000. Mental health problems surged, including deaths by suicide. The increases to SAMHSA’s budget had bipartisan support.

Now there is widespread talk that the Trump administration may fold SAMHSA into another health agency or return staff numbers and grant funds to 2019 levels, even though rates of overdose deaths remain significantly higher than in 2019. According to the most recent C.D.C. update, between September 2023 and September 2024, roughly 87,000 people died of drug overdoses.