Elon Musk and his supporters offer several explanations for his contrarian views, unfiltered speech and provocative antics. They’re an expression of his creativity. Or the result of his mental-health challenges. Or fallout from his stress, or sleep deprivation.

In recent years, some executives and board members at his companies and others close to the billionaire have developed a persistent concern that there is another component driving his behavior: his use of drugs.

And they fear the Tesla and SpaceX chief executive’s drug use could have major consequences not just for his health, but also the six companies and billions in assets he oversees, according to people familiar with Musk and the companies.

The world’s wealthiest person has used LSD, cocaine, ecstasy and psychedelic mushrooms, often at private parties around the world, where attendees sign nondisclosure agreements or give up their phones to enter, according to people who have witnessed his drug use and others with knowledge of it. Musk has previously smoked marijuana in public and has said he has a prescription for the psychedelic-like ketamine.

In 2018, for example, he took multiple tabs of acid at a party he hosted in Los Angeles. The next year he partied on magic mushrooms at an event in Mexico. In 2021, he took ketamine recreationally with his brother, Kimbal Musk, in Miami at a house party during Art Basel. He has taken illegal drugs with current SpaceX and former Tesla board member Steve Jurvetson.

People close to Musk, who is now 52, said his drug use is ongoing, especially his consumption of ketamine, and that they are concerned it could cause a health crisis. Even if it doesn’t, it could damage his businesses.

Illegal drug use would likely be a violation of federal policies that could jeopardize SpaceX’s billions of dollars in government contracts. Musk is intrinsic to the value of his companies, potentially putting at risk around $1 trillion in assets held by investors, tens of thousands of jobs and big parts of the U.S. space program.

SpaceX is the only U.S. company now approved to transport NASA astronauts to and from the International Space Station. The Pentagon, meanwhile, has stepped up purchases of SpaceX rocket launches in recent years, and the company has also been looking to develop a large business selling satellite services to national-security agencies.

One former Tesla director, Linda Johnson Rice, grew so frustrated with Musk’s volatile behavior and her concerns about his drug consumption that she didn’t stand for re-election to the electric-car company’s board in 2019, according to people familiar with the matter.

Musk didn’t respond to requests for comment.

An attorney for Musk, Alex Spiro, said that Musk is “regularly and randomly drug tested at SpaceX and has never failed a test.” Spiro, who said he represents Tesla, added in response to detailed questions that “there are other false facts” in this article but didn’t detail them.

The people around Musk long ago became accustomed to his volatile behavior. Some SpaceX executives who had long worked with him, however, noticed a change at a company event in late 2017.

Hundreds of SpaceX employees gathered around mission control at the rocket company’s headquarters in Hawthorne, Calif., in anticipation of Musk, who was nearly an hour late to arrive at the all-hands meeting about the company’s latest rocket.

When he finally took the stage, Musk was strangely incomprehensible at times. He slurred his words and rambled for around 15 minutes, according to executives in attendance, and referred repeatedly to SpaceX’s Big Falcon Rocket prototype, which was known as BFR, as “Big F—ing Rocket.”

SpaceX President Gwynne Shotwell ultimately stepped in and took over the meeting.

It couldn’t be learned if Musk was under the influence that day. But after the meeting, the SpaceX executives privately talked about their worries Musk was on drugs. One described the event as “nonsensical,” “unhinged” and “cringeworthy.”

Spiro called the description of the SpaceX incident “false as has been confirmed by countless people who were present.” He declined to elaborate on what specifically was false or describe the countless people.

Then in 2018, people familiar with Musk’s behavior said, another incident seemed to mark a turning point for him—and showed that his drug use could have consequences for his businesses. That year, Musk got into trouble with NASA for smoking marijuana on the Joe Rogan show, raising red flags for some about the business impact of Musk’s conduct and causing employees at SpaceX to be randomly tested for drugs.

Worry by board members

In addition to violating federal contracts, any kind of illegal drug use would break company policies at both SpaceX and Tesla, and would raise questions about Musk’s executive role at the publicly traded Tesla, where the board has a duty to shareholders to oversee management.

Some Tesla board members over the years have talked among themselves about their concerns over Musk’s alleged drug use but haven’t said anything formally that would end up as an official board agenda item or in meeting minutes, people familiar with the discussions said. Some directors, including current Tesla board chair Robyn Denholm, have gone to Kimbal Musk, who is a Tesla board member and was a SpaceX board member until early 2022, for help with Musk’s behavior, without using the word “drugs,” the people said.

Some on the board and others close to Musk worried he was on drugs when he tweeted in 2018 about plans to take Tesla private, people familiar with the episode said. Kimbal Musk informally approached Musk about it on behalf of some board members, some of the people said. The tweet brought on an SEC investigation into whether the statement was misleading or false, and resulted in Musk’s agreement to step down as Tesla chairman for a time.

Some close to Musk said they learned he was under the influence during a media interview he gave soon after the tweet when he choked up describing how difficult his year had been.

Part of the issue directors have grappled with over the years is whether drug use by Musk is to blame for his unusual behavior, or if it is something else, such as his consistent lack of sleep, which he has talked about.

Musk oversees six companies, including social-media platform X, formerly Twitter; his tunneling venture, The Boring Co.; his brain implant startup, Neuralink; and a new artificial-intelligence company, xAI. His business life bleeds into his personal time in a way that is uncommon even for other chief executives.

At Tesla and X, he has said he regularly slept at the office. He often emails company lieutenants in the middle of the night, and hosts work meetings at midnight. He said he works almost nonstop. “Vacation is a strong word,” he said in 2022 court testimony. “For me, it is email with a view.”

In a new authorized biography of Musk, author Walter Isaacson described Musk’s “demon mode”: Musk, Isaacson wrote, entered into a state of intense fury and would frequently lash out at employees and executives. In the Isaacson book, Musk is quoted as saying, “I really don’t like doing illegal drugs.”

Musk and others have attributed his erratic office behavior to his mental health. A Twitter user asked in 2017 if he had bipolar disorder, which can cause mood swings, to which Musk replied affirmatively, although he said he was undiagnosed.

Musk also said, when he hosted “Saturday Night Live” in 2021, that he had Asperger’s, a form of autism.

Drug use has also been a thorny topic for directors at Musk’s companies because some of them are his close friends, and attend parties and travel with him, according to people familiar with the matter and court documents.

Musk has been known to attend parties and events at Burning Man, the Nevada arts and music festival where drugs are widely used, to blow off steam, according to people close to him.

He also throws his own private events, where drug use is common, according to people who have attended the parties.

The executive’s whereabouts are often a closely guarded secret, and he is protected by private security. At his companies, executives and employees often have to sign nondisclosure agreements.

Kimbal Musk frequently attends the same parties and events as his brother, including an Art Basel party in Miami in late 2021 when both took ketamine recreationally, according to people with knowledge of their drug use. Kimbal Musk has talked to friends about the benefits of psychedelics, and tweeted in June in support of his wife when she spoke at the country’s largest psychedelic conference about the benefits of the drugs for mental health treatment.

In 2023, The Wall Street Journal reported that Elon Musk microdoses ketamine for depression and also takes full doses at parties. Following publication of the article, Musk tweeted that ketamine is a better way to deal with depression compared with more widely prescribed antidepressants that are “zombifying” people.

In a separate tweet, Musk later said that he had a prescription for ketamine. The psychedelic-like drug can be prescribed “off label” for depression and post-traumatic stress disorder, and it is also a widely used party drug that can be purchased illegally through dealers.

Government contracts

As a general rule, board members and executives have long struggled with how to deal with substance abuse at their companies.

Some directors have wondered whether it is their role to police drug use outside the office, saying what executives do in their personal lives—especially with drugs that may be legal in certain instances or states—may not impact their business decisions.

At most companies, board members aren’t required to investigate an executive for drug use, but they often do take action if they believe it is impacting the business.

That can include encouraging a leave of absence for treatment or opening an investigation, corporate governance experts say. Drug use is a more challenging issue than other substance problems, such as alcohol, because possessing certain drugs, such as cocaine, can bring felony charges.

In 2020, former Zappos chief executive Tony Hsieh’s abuse of ketamine ultimately caused executives at the shoe company’s parent, Amazon, to step in, according to people familiar with those events.

Amazon executives gave Hsieh a couple of months to clean up his act, and when he wasn’t able to do so, he resigned. Hsieh was trapped in a house fire while under the influence in late 2020 and later died from his injuries.

Tesla’s code of conduct described the electric-vehicle maker as a drug-free workplace and prohibits all employees, including executives, from using them, even out of the office.

Illegal drug use by employees is also in violation of the rules that govern the more than $14 billion in contracts that Musk’s private rocket company, SpaceX, has with the U.S. government for civilian and military space missions.

Federal contracts require that companies comply with the Drug-Free Workplace Act and foster a drug-free culture with programs and policies, regardless of any state laws that may legalize some usage. Contractors can also lose security clearances because of drug abuse, defined as the use of illegal drugs or prescription medications “in a manner that deviates from approved medical direction.”

In his role as CEO and founder of SpaceX, Musk has a security clearance that gives him access to classified information.

Investors have often turned a blind eye to concerns about Musk, including his drug use, especially when Tesla is doing well, investors and people close to the board said. And in recent years, Tesla and SpaceX have both performed exceptionally. Tesla stock is up around 1,000% in the past five years, despite declines in 2022, compared with the S&P 500’s increase of around 86% in that time period. Revenue at SpaceX also has soared.

Tweet firestorm

In the summer of 2018, some people around the billionaire began getting concerned that he was losing control.

Several months after the all-hands meeting at SpaceX, Musk tweeted in August that he planned to take Tesla private at $420 a share—“420” is slang for smoking marijuana—and that he had “funding secured.”

The now-infamous tweet set off a firestorm among investors, who scrambled to understand the billionaire’s plans for the electric-car maker. Tesla shares increased more than 6% the day of the tweet. The SEC launched an investigation that resulted in a settlement that included fines of $40 million and Tesla adding two independent board members and overseeing the CEO’s communications. Musk didn’t admit or deny wrongdoing.

Board members told regulators they didn’t know about Musk’s plans and were taken aback by his actions. Privately some on the board became concerned that Musk was on drugs when tweeting, and some directors briefly discussed among themselves the idea of him taking a leave of absence from Tesla, people familiar with the discussions said.

In an interview with the New York Times soon after, Musk choked up multiple times as he described the intense personal toll of leading Tesla, saying, “This past year has been the most difficult and painful year of my career.”

Behind the scenes, things were much more dire: Musk had been under the influence as he answered the reporters’ questions, according to people briefed on the episode. The CEO hadn’t informed Tesla’s communications team that he was giving the interview, people familiar with the episode said.

Soon after, Musk smoked marijuana on the comedian Rogan’s show, which can be streamed online. NASA demanded written assurances that SpaceX was complying with the federal drug-free workplace law and spent $5 million in taxpayer dollars on training for SpaceX employees, according to a letter NASA sent to the company and federal contracting records.

Musk has said the agency required drug testing at SpaceX for a year.

Corporate contractors must follow standard NASA guidelines for drug tests that usually check for marijuana and cocaine and have the ability to also test for amphetamines, opiates and PCP.

Spiro didn’t respond to a question on what type of drug tests Musk has taken.

At SpaceX and some of Musk’s other companies, including the tunnel venture Boring Co., executives began warning employees to follow company rules at all times, including to not use illegal drugs, even out of the office, according to people familiar with the warnings.

SpaceX brought in drug-sniffing dogs on a random basis to make sure employees weren’t carrying illegal substances, according to the people.

At Tesla, Denholm, the current board chair, James Murdoch and other directors sometimes gathered around Kimbal Musk informally during board breaks or after meetings to ask how Elon Musk was doing or if he was getting enough sleep, people familiar with the conversations said. While the directors wouldn’t specifically ask about substance abuse, the people said they understood the questions to be about Elon Musk’s perceived drug use.

Rice, the director who didn’t stand for re-election in 2019, raised concerns about his drug consumption more than once in side conversations with board members about Musk’s increasingly erratic behavior during her two year tenure on the board, according to people familiar with her concerns. She informally asked whether the board should investigate and was brushed off, one of the people said.

—Joe Palazzolo, Micah Maidenberg, Jim Oberman and Jeff Horwitz contributed to this article.

Write to Emily Glazer at Emily.Glazer@wsj.com and Kirsten Grind at kirsten.grind@wsj.com