Two 45-minute daily workouts. One gallon of water. 10 pages of a nonfiction book. A diet. No “cheat meals” or alcohol. For 75 days.

And if you mess up, you have to start from the beginning.

Sound like a lot? It’s supposed to be. The program, called 75 Hard, is meant to build mental toughness. Some say that rigidity is what makes it great, and others say that makes it problematic.

Since it was created in 2019, 75 Hard has developed somewhat of a cult following, with practitioners posting daily progress pictures and videos that sometimes rack up millions of views on TikTok and Instagram. One of Reddit’s biggest subreddits, with over 44,000 members, is dedicated to the program.

But is it beneficial, and are the changes sustainable? Psychologists say that while the program can have mental-health benefits, certain vulnerable groups may be pushing themselves too far without benefit. Exercise experts also say the regimen could be too taxing for those who aren’t young and active already, and could lead to physical injury.

“It can sound really cool and exciting and helpful, but is this something that’s actually really ultimately helpful, sustainable, good for the person?” asked Dr. Thea Gallagher, a clinical psychologist and the director of wellness programs at New York University.

“It would be great to have more continued rigorous research around these exciting programs-slash-challenges,” she said.

Andy Frisella, the creator of 75 Hard and a motivational speaker, encourages people to speak with a medical professional before starting the program. His team did not respond to a request for comment.

Why do people do 75 Hard?

According to Mr. Frisella, who said in a 2022 episode of his podcast that he spent 20 years developing 75 Hard, tens of thousands of people have completed the program, which is meant to help people build resilience, grit and perseverance, among other traits.

“This is the equivalent of an Iron Man, of climbing Mount Everest,” Mr. Frisella said on the podcast. “Whatever it is that you see all of these other people doing that they’re so proud of — this is the equivalent of that for your brain.”

People who have completed the program have said on social media that it helped them improve their confidence, lose weight, try new workouts and follow through on what they set out to do. Many complete it in the first 75 days of the year, while others start it whenever they need a reset.

Outdoor workouts, no ‘cheat meals,’ no alcohol

What’s hardest about the program varies from person to person. But many have balked at the requirement of two daily 45-minute workouts, and the avoidance of “cheat meals” — meaning deviating from any diet you have chosen for yourself — and alcohol for the duration of the program.

Mr. Frisella has explained that the workouts can be any level of intensity — even a walk. At least one of the two daily workouts must be completed outdoors.

One participant on TikTok went on an outdoor walk during a blizzard, another completed a strength training workout in the rain, while another jumped rope for 45 minutes outside at night. Others varied their indoor workouts by alternating between running, strength training, yoga and more.

By going outdoors, the program enforces the lesson that “the conditions are not always going to be perfect,” Mr. Frisella said on a 2019 episode of his podcast.

The daily workouts must be spaced out by at least three to four hours.

The program notably lacks any built-in rest days.

The program also insists that participants follow a diet — for example, a vegetarian, vegan, or ketogenic diet — but Mr. Frisella does not offer much guidance on what it should be, only that people should choose “a diet that is going to improve your physical health.”

Participants must follow the diet they choose without deviation, or else restart the program.

Alcohol is strictly forbidden.

Achieving tasks can build confidence

“Something like this could enhance someone’s confidence or their mental fortitude,” said Dr. Kate Gapinski, an adjunct professor of psychology at the University of San Francisco.

“When you see that you’re able to complete something so difficult, and actually sustain it for 75 days, which is quite a long time for a significant habit change, I could see that inspiring confidence about other future difficult tasks,” she said.

The program advances certain behaviors that psychologists encourage their patients to adopt.

The tasks that can be completed quickly — namely the 10 pages of reading a nonfiction book — are exactly the kinds of bite-size tasks that experts say may encourage people trying to enact change in their lives.

But challenges can arise when tasks are too big or feel unsustainable. “If you do something that takes a lot of energy-slash-motivation, commitment, the problem is that when you don’t succeed at doing that, sometimes people end up feeling demoralized and worse than when they started,” Dr. Gallagher said.

Is the rigidity of the program harmful?

Some participants take the program very seriously. The program “is hard for a reason,” one poster wrote on the subreddit. “If you don’t like that, go somewhere else or at the very least, don’t be mad when people call you on your modifications to the program.”

But several health experts had concerns about such strict regimens.

The workout requirements could be worrisome for inactive or frail people, said Patrick J. O’Connor, a professor of kinesiology at the University of Georgia.

“Ninety minutes per day, that would be excessive for some people and it could produce injuries for some people,” he said. “A lot of times, the biggest risk for injury is if somebody goes from very little, to quite a bit.”

Mr. O’Connor pointed out that the program altogether called for 630 minutes of exercise each week — that’s more than four times the amount recommended by federal officials, which is 150 minutes of “moderate-intensity physical activity” and two days of strength training.

There are also concerns about the mental health ramifications of such a no-exceptions program.

“I wouldn’t recommend the program for people with an active eating disorder,” said Dr. Gapinski. “With eating disorders, we’re actually trying to expand comfort about which kinds of foods are consumed,” she said, adding that moderation is emphasized in treatment.

It may be more helpful for people to find small tasks that are meaningful to them instead of choosing a prescriptive program, said Dr. Alexandra Gold, a clinical psychologist at Massachusetts General Hospital and Harvard Medical School.

“I think if someone is just given a prescription of ‘Oh, you do these things,’ it’s not necessarily originating from them, and that’s a big factor also in consistency and sustainability,” Dr. Gold said.

Perhaps unsurprisingly, a number of modified versions of the plan have emerged, including 75 Soft. In that version, the water requirement is lower and just one 45-minute daily workout is required.