“You might be working with a situation where, with an addictive individual, you could have some good years and some bad years,” Ms. Francois said. “You want that latitude to make changes with the distributions.”

People can also use these trusts to shield siblings from feeling bound to care for a troubled sister or brother. Most of the trusts Ms. Francois’s firm works with, she said, are worth more than $5 million. Below that, the costs to administer them are too high for the firm.

Beyond the large sum of money needed to establish a professionally managed trust, which in itself excludes many families, special-purpose trusts are more complicated to establish than regular trusts because of the powers they give to trustees and the restrictions they place on distributions. But advisers say the hardest part about setting up these trusts can be getting parents to accept them as necessary. Doing that means parents have to acknowledge that their children are never going to fully recover.

“It requires the parents and often the grandparents to recognize there’s an issue out there,” said Robert M. Freedman, a partner at the law firm Schiff Hardin who has worked on many such trusts. “The recognition of the issue is a lot tougher with this group than with someone who is developmentally disabled, like with autism. You see it at birth and plan accordingly.”

He said most people came to him after their children had dropped out of school in their late teens or 20s. He attributes the hesitancy to the stigma around some forms of mental illness like schizophrenia and around addiction to drugs like heroin.

“It’s easier to say my son isn’t graduating from medical school, he’s just coming home for a while,” Mr. Freedman said.

But if parents do that, Mr. Freedman said, the estate plan they put in place for all of their children could actually harm the troubled child who would then have the same access to funds as the other siblings.