DETROIT – In a small, second-story office on Main Street in Ann Arbor, Mich., Liberty Clinic is doing brisk business, selling medical marijuana for $360 to $400 an ounce. In just 3½ months, 750 patients have come through its doors.

In Lansing, Mich., Danny Trevino has expanded beyond his HydroWorld hydroponics store, adding two medical clinics, grow classes and a dispensary.

And in Ypsilanti, Mich., Darrell Stavros and his partners have set up a medical marijuana service center, renting space to a support group, doctors and a bong shop. “This is creating an enormous amount of businesses that never existed,” he said.

Medical marijuana, one of Michigan’s newest industries, is taking off. Dozens of hydroponics stores, medical clinics and grow schools are popping up. And at support groups, cafes and dispensaries, patients and growers are buying and selling the drug.

As with any industry, there are challenges, such as crop failures and theft. And limits on the size of growers’ crops make it all but impossible for growers to get rich, though they can earn some decent money.

“A few people will make a few bucks. Most people won’t make much,” said Adam Brook, organizer of the annual Ann Arbor Hash Bash.

In Michigan’s burgeoning medical marijuana industry, few rules exist, much of the business occurs in secrecy and the only way for growers to make big bucks is to break the law.

“If you operate within the law, you’re not going to make a lot of money,” said Leili Russo, who grows marijuana for medical purposes and serves as the secretary of the Genesee County Compassion Club in Flint, Mich.

Growers, also called caregivers, say that at best, they can make $40,000 a year. And that’s after spending $1,000 or more on equipment and other supplies, and putting in countless hours every day tending to plants.

Under Michigan’s medical marijuana law, caregivers can supply only five patients. Each patient can have 12 plants. But growers who choose to ignore these rules can easily make $100,000, said Brook, an industry consultant, at an annual rally to support reforming marijuana laws.

With these conditions, it’s no surprise that medical marijuana is becoming a big business in Michigan’s depressed economy. Nineteen months after residents voted to legalize medical marijuana, the industry has attracted more than 8,000 caregivers, people who grow and harvest marijuana plants so they can be turned into medicine for patients, according to the Michigan Department of Community Health.

For caregivers who abide by the law, this kind of work is usually a second job. That’s the case with Corey Hathaway, 33, of Eaton Rapids, Mich. Hathaway used to run his own commercial construction company, but that business dried up when the economy tanked. So he found a job working at HydroWorld, a hydroponics shop in Lansing. To supplement his income, he also is a caregiver with five patients.

“The people that are greedy don’t succeed because they can’t maintain the patient-caregiver relationship,” he said.

The law is vague about what caregivers can do if they produce more marijuana than their patients need. To make extra money, some sell their overages on the black market or to dispensaries, clinics or other caregivers.

Growing marijuana is just one part of the rapidly expanding industry. Experts say more lucrative opportunities can be found selling the hydroponic equipment that caregivers need and teaching them how to grow marijuana properly.

Another moneymaker: operating clinics that help people get the paperwork they need to qualify as medical marijuana patients.

These kinds of service businesses are springing up all around the state and are the most visible part of the industry. Already, price wars have sprung up among the dozens of hydroponics shops that have opened in southeast Michigan.

Hydroponics stores aren’t the only ones cashing in. Attorneys, grow consultants, grow-room designers and contractors and grow schools are all finding a market for services.

“There are so many people that are excited about being able to work,” said Michael Komorn, a Southfield, Mich., medical marijuana attorney and the treasurer of the 17,000-member Michigan Medical Marijuana Association. “They want to get back into the marketplace.”

Entrepreneurs also are flocking to the sales side of the business, operating an estimated 20 dispensaries, cafes and clinics in the state, according to medical marijuana attorneys.