“Is it the government’s position that if I unlawfully use Ambien or I unlawfully use Xanax, then I become dangerous?” Justice Amy Coney Barrett asked a lawyer for the Trump administration, which is arguing to uphold the current law. “What is the government’s evidence that using marijuana a couple times a week makes someone dangerous?” she added.

It was not immediately clear how the justices might handle those concerns. One option might be to require that the government prove that a drug user is an addict to obtain a conviction under the statute. But several members of the court voiced concern with that possible approach, fearing it could lead to chaos in lower courts, requiring judges to parse out the drug habits of people charged under the law.

The case, the second major gun rights dispute before the justices this term, has also drawn interest because the law is the same statute used to convict Joe Biden’s son, Hunter, in 2024.

The case may give the court an opportunity to clarify how to apply a test the justices set out in a landmark 2022 Second Amendment ruling. It requires courts to analyze whether gun laws align with the country’s “history and tradition” of firearms regulation to determine if they are constitutional. Lower courts have struggled with how to apply the test.

The question before the justices involves the constitutionality of a section of the Gun Control Act of 1968, legislation enacted as a response to the assassinations of Robert F. Kennedy and the Rev. Martin Luther King Jr. Amended in the 1980s, the law bans gun possession by anyone who “is an unlawful user of or addicted to any controlled substance.”

The law aimed to create a broad framework to keep guns away from people whom Congress deemed irresponsible and dangerous.

The new test led to debates over founding-era laws that restricted the rights of people who were “habitual drunkards.” The Trump administration argued that such laws meant the gun control statute was rooted in the nation’s historical tradition.

But a lawyer for Ali Hemani, a Texas gun owner who challenged the law when he was arrested after telling law enforcement that he used marijuana, pointed to a difference in such laws and Hemani’s situation.

The lawyer, Erin E. Murphy, explained that those laws focused on whether someone was “drinking to such excess that they can’t care for themselves or their affairs,” or was “posing a public safety risk,” not on people who merely used alcohol.

Justice Neil Gorsuch pointed out that there might have been a much higher bar for what it meant to overuse alcohol at the time of the country’s founding.

“John Adams took a tankard of hard cider with his breakfast every day,” Gorsuch said. “James Madison reportedly drank a pint of whiskey every day. Thomas Jefferson said he wasn’t much a user of alcohol; he only had three or four glasses of wine a night, OK?”

He asked the Trump administration lawyer, Sarah M. Harris, a principal deputy solicitor general, whether those founders were “all habitual drunkards who would be properly disarmed for life.”

Later in the argument, Chief Justice John Roberts appeared to raise a concern that limiting the law to people who are addicted to drugs could require judges to assess drug use “on a case-by-case basis” and an “individual-by-individual basis.”

The lively argument in US v. Hemani lasted about two hours and led to colorful hypotheticals about the use of all manner of drugs, including cough syrup, sleep gummies, and psychedelics.

Justice Elena Kagan probed how courts should handle drug use for substances that impair users, but that might not be addictive.

She used a hypothetical about ayahuasca, a psychoactive brew from the Amazon that causes hallucinations, saying that “when you’re in its grip, you basically — reality dissolves, all right?” Should the government not be able to restrict a person who uses the drug but is not addicted to it, she asked Hemani’s lawyer.

Murphy said it could depend on “how lasting the impact is.”

In early August 2022, federal agents arrived at the suburban Texas home of Ali Hemani, a former high school football player whose family had come under scrutiny because of its ties to Iran.

During a search of the family’s house, Hemani told agents that he kept a handgun locked in a safe. He also told them that he used marijuana “about every other day,” pointing them to about 60 grams of marijuana in the house. Agents also found cocaine in his parents’ closet.

Months later, the FBI arrested Hemani, accusing him of violating a federal law that bars drug users and addicts from owning or possessing guns.

The case has scrambled typical political alliances. The Trump administration is defending the law. It has been supported in briefs by Everytown for Gun Safety, a group backed by former New York City Mayor Michael Bloomberg, a Democrat, along with leaders of several Democratic-led states, including California, Hawaii, and Massachusetts.

Hemani is backed by an unusual coalition of the American Civil Liberties Union, a law school clinic that specializes in helping Muslims affected by counterterrorism policies, the National Rifle Association, and the Drug Policy Alliance, which advocates decriminalizing drug possession. Hunter Biden, who was pardoned by his father, told The New York Times he also supports Hemani’s bid.

The investigation into Hemani did not begin with his marijuana use. Instead, the Trump administration says that officials searched Hemani’s phone at a border crossing in 2019 and became suspicious. The administration claims the officials found communications showing “he was poised to commit fraud at the direction of suspected affiliates of the Iranian Revolutionary Guard Corps, a designated foreign terrorist organization.”

Law enforcement also flagged Hemani’s regular contact with his brother, who attended an Iranian university that Trump administration lawyers say the US government has designated as having ties to terrorism. Plus, they assert they discovered that Hemani was “a drug dealer who uses illegal drugs.”

Three years after searching his phone, the FBI went to Hemani’s home, where they found the Glock 9 mm pistol, marijuana, and cocaine. Hemani told them that he used marijuana and agreed that the cocaine was his.

Six months later, federal prosecutors charged Hemani with one count of possession of a firearm by an “unlawful user” of a controlled substance based on his marijuana use.

Hemani’s lawyers have strongly disputed the government’s description of events. In briefs to the court, Hemani’s lawyers describe him as a US citizen, born and raised in Texas, a high school honor student and football player who graduated from the University of Texas at Arlington before going into project management.

The government, they assert, made “‘terrorism’-related insinuations about Hemani and his family based on their religious and ethnic identities,” but they note that neither Hemani nor any member of his family was ever charged in connection with those allegations.