A study of more than 30,000 British adults diagnosed with attention deficit hyperactivity disorder, or A.D.H.D., found that, on average, they were dying earlier than their counterparts in the general population — around seven years earlier for men, and around nine for women.

The study, which was published Thursday in The British Journal of Psychiatry, is believed to be the first to use all-cause mortality data to estimate life expectancy in people with A.D.H.D. Previous studies have pointed to an array of risks associated with the condition, among them poverty, mental health disorders, smoking and substance abuse.

The authors cautioned that A.D.H.D. is substantially underdiagnosed and that the people in their study — most of them diagnosed as young adults — might be among the more severely affected. Still, they described their findings as “extremely concerning,” highlighting unmet needs that “require urgent attention.”

“It’s a big number, and it is worrying,” said Joshua Stott, a professor of aging and clinical psychology at University College London and an author of the study. “I see it as likely to be more about health inequality than anything else. But it’s quite a big health inequality.”

The study did not identify causes of early death among people with A.D.H.D. but found that they were twice as likely as the general population to smoke or abuse alcohol and that they had far higher rates of autism, self-harming behaviors and personality disorders than the general population. In adulthood, Dr. Stott said, “they find it harder to manage impulses, and have more risky behaviors.”

He said health care systems might need to adjust in order to better serve people with A.D.H.D., who may have sensory sensitivity or difficulty managing time or communicating with clinicians during brief appointments. He said he hoped treatments for substance abuse or depression could be adapted for patients with A.D.H.D.

“If it’s about systems, it’s malleable,” he said. “This doesn’t have to be.”

Previous studies have pointed to an unusual number of early deaths for people with A.D.H.D. A 2022 meta-analysis in the journal JAMA Pediatrics found that deaths from unnatural causes, such as accidents or suicide, were 2.81 times higher among those diagnosed with A.D.H.D. than in the general population.

A 2019 study that used actuarial tables to predict life expectancy concluded that adults diagnosed with A.D.H.D. in childhood had an 8.4-year reduction in life expectancy compared with the general population, something the authors attributed to reduced education and income, higher rates of smoking and alcohol consumption and reduced sleep.

Russell Barkley, the lead author of that study, said the data made it clear that A.D.H.D. should not be seen as a childhood disorder, like bed-wetting, but as a lifelong problem.

“To me, the best analog is diabetes,” said Dr. Barkley, a retired professor of clinical psychology at Virginia Commonwealth University. “This is a disorder that you’ve got to manage, like high blood pressure, like cholesterol and diabetes. You’ve got to treat this for life.”

The new study examined 9,561,450 patients in Britain’s National Health Service primary care practices, among whom 30,039 had been diagnosed with A.D.H.D. Each person in the A.D.H.D. group was matched with 10 peers without the disorder for the purposes of comparison. Among those with A.D.H.D., 193 male patients and 148 female patients died during the follow-up period, which lasted from 2000 to 2019.

Stephen Hinshaw, a professor of psychology at the University of California, Berkeley, who studies A.D.H.D. but was not involved in the new British research, described the study as “a major finding,” the first analysis of deaths of subjects diagnosed with the disorder. He said it was unfortunate that the subjects’ causes of death were not included.

“There are risk factors to work on,” he said. “That is the key limitation of the study, because it would be really important to know, in terms of prevention, should we be focused on suicidality? Better diet and exercise? Depression?”

Diagnosis of A.D.H.D. has shifted in recent years, as clinicians concluded that many older people, especially women and people of color, had gone undiagnosed early in life and could benefit from treatment. As first-time diagnoses surged among older people, prevalence has remained consistent among children, at around 11 percent in the United States and 5 percent in Britain.

Dr. Stott said he hoped that, as these demographic changes took hold, health care systems would make more effort to identify the needs of neurodivergent patients. In past decades, he said, they may have been viewed dismissively by caregivers, as “the naughty kid at school.”

“If you’re constantly told, as a kid, sit down, stop being so naughty — if you talk to people with A.D.H.D., that is their experience — stop talking, go and sit outside,” he said. “It’s all of these things that wear down at your life chances, in many ways.”