"I smoked pot once in high school," he said. "I don't smoke pot and never will. But if there are people who are suffering and this can help them, why not make it available?"
Look closely and you can spot the resemblance to his billionaire brother, Mark. At 6-2 and 230 pounds, Brian is a big guy. And, like Mark, he speaks loudly and passionately about his beliefs.
"This is not about Cheech and Chong," he said.
"We're not talking about pot shops cropping up along Northwest Highway like strip clubs. We need to get the attention of the suits and ties, and the soccer moms."
But medical marijuana is only one of his passions.
As a lawyer, he takes on hate speech in a scholarly new book called You Don't Say: Speaking Freely in an Era of Hatred . He runs the Fallen Patriot Fund, a charity for the families of soldiers killed or wounded in the Iraq war. He writes a blog called The Cuban Revolution, which covers subjects such as women who get "tramp stamp" tattoos at the small of their backs. And he runs marathons to stay fit.
"Brian moves to his own drummer," said older brother Mark, the 52-year-old owner of the Dallas Mavericks.
"He finds things that interest him, and he goes after them."
At age 49, Brian Cuban is still very much a work in progress.
He wants to become an accomplished writer and host a talk show. He pays the rent as executive director of the Mark Cuban Foundation, which supports the Fallen Patriot Fund.
Cuban drives a 2007 gray Lexus and lives in The Renaissance, a townhome and apartment complex just west of NorthPark Center. He's been married and divorced three times. His dog, Peanut, and cat, Useless, share his home. His girlfriend of 4 1/2 years lives in the same complex. She keeps him company and helps him stay sober.
"I don't want to be in a story if it's going to be about drugs," she told The Dallas Morning News. "If you use my name, I'll pitch a fit."
Cuban is an open book about his addictions and his struggle to stay clean with a customized 12-step program that focuses on personal responsibility instead of dependence on a higher power. He describes himself as "culturally Jewish" but not religious.
"It is possible for an addict to say he's gonna change his life and do it," Cuban said. "You can use the concept of personal responsibility to change."
Cuban's candor is disarming and was on full display recently during a lunch at R+D Kitchen, a favorite North Dallas hangout near the intersection of Preston Road and Northwest Highway.
Between bites of a sushi roll and couscous, he told a story featuring himself as an overweight 15-year-old growing up in Pittsburgh. Often, he walked home after school with his brother Jeff, who was 12. As a protective older brother, Brian sometimes held Jeff's hand.
Painfully shy, Brian forced himself to attend a Sadie Hawkins Day dance at the school. As he stood by himself, a girl approached. He thought she was going to ask him to dance. Instead, she said, "Why do you hold your brother's hand when you walk home from school? That's weird!" And she walked off.
"That moment has affected me to this day," he said. "I don't think I had a date until I was 24."
Jeff Cuban, now 46, lives in Dallas and works as executive vice president of HDNet, a cable television channel owned by brother Mark.
"Brian was very studious as a young man," Jeff Cuban said. "With our age difference, we didn't spend much time together during childhood. We only started hanging out in our adult lives."
Brian and Jeff followed Mark to Dallas in the early 1980s. Their father, Norton Cuban, 85, has lived in Dallas for 10 years. He expresses no surprise that his middle son has embraced legalization of marijuana for medical purposes.
"Brian is open to everything," Norton said. "He's human. And he puts a lot of thought into what he's doing. We all support him in what he does."
Shirley Cuban, Norton's wife and mother of his three sons, still lives in Pittsburgh.
Brian Cuban got his law degree and went to work in the insurance industry. Later, he handled cases for plaintiffs referred to him by the NAACP in Oklahoma.
Finally, he went to work for Mark Cuban in 1999. He remembers becoming instantly popular. He calls it "fame by name." Women paid attention to him. He bought expensive clothes, including a $1,500 pair of two-tone, red and black alligator shoes (size 14). He still wears them.
"I would never make such a purchase today," he said. "But it would be a waste not to wear them."
He hung out at all the right clubs -- Sense, Medici, Martini Ranch, ZaZa, Ghost Bar, Beau Nash. He started drinking, which loosened him up enough to try cocaine and then begin using it. He loved it. And he hated it.
"All of a sudden, I thought I was somebody," Cuban said. "I was a hologram. I was like a kid in the candy store. I'm not blaming anyone. I was not secure in who I was, so I had to create someone new."
Somehow, Cuban hid his addiction from longtime friends and colleagues. He maintained "party friends" in one compartment and "regular friends" in another.
Angelo Marchese, a Dallas businessman and longtime friend, said he and Cuban generally hang out with conservative people. He said he never knew Brian was doing drugs during those years.
"I guess I couldn't see the forest for the trees," Marchese said. "Once you are my friend, you will always be my friend. And he is the same way."
Seven years of partying passed into history.
"I look back at those seven years, and I did nothing," Cuban said.
But he stayed close to family. To this day, he eats lunch every Tuesday with Jeff and their father. He's been sober since early 2007. And he's now an old hand at deflecting people who ask him to pitch a business proposition to Mark.
"I'm always polite about it," he said with a smile. "I just say, 'You'll have to talk to Mark about it.' "
Fifteen states have legalized some form of marijuana use for medical purposes. Typically, the law requires a doctor to recommend the treatment for a patient.
In Colorado, for example, a patient with a doctor's recommendation can legally possess up to 2 ounces of marijuana or grow up to six plants for his own use.
New regulations governing commercial marijuana dispensaries storefronts that sell marijuana products are going into effect in Colorado. Patients there now have the option of buying their medicine from an authorized dispensary rather than getting it from a legally questionable dealer or growing their own pot.
Cuban describes himself as a political centrist, neither right nor left. He voted for Republican John McCain for president in 2008. As a longtime resident of red state Texas, he's under no illusion that the Legislature, which convenes in January, will pass a medical-marijuana law.
"A reasonable goal would be to get something seriously considered by the Legislature by 2013," he said.
Dante Picazo, a Dallas businessman, approached Cuban about becoming an advocate for medical marijuana in Texas. Earlier this year, Picazo incorporated the for-profit Medcan University, a traveling seminar that educates its "students" about the industry.
Medcan (short for medical cannabis) held its first training workshop recently in a Fort Worth restaurant. Cuban's role was to present his analysis of state and federal laws pertaining to marijuana. An estimated 20 students, each paying $250, attended.
"People are scared," Cuban said. "It's tough getting people to come to the seminar. But it's not illegal to talk about marijuana."
The harshest critics of the medical-marijuana movement especially the industry as it developed in California say it's just a way for people to get high legally.
But Picazo and Cuban say making marijuana available to sick people who need it is a humanitarian issue. And it is a potential source of new tax revenue for state and local government budgets.
"The reason I chose Brian to be a part of Medcan is that he is a person in the world with a gigantic, Class A heart," Picazo said. "Brian is always for the underdog, and he is for people who don't have much of a chance in life.
"If you are not as good-looking or you're not incredibly well-put-together and connected and perfect, Brian is the one who will pay attention to you."