ADHD is just a short hand for "mild brain impairment/damage of unknown cause"
It's not a single disease but a collection of dysexecutive syndromes that impact higher cognitive functions: strategic planning, organization, etc.
This could be due to low oxygen during birth, infections (not necessarily CNS invasive), neck/head injuries, chronic pain, sleep disorders, genetics, vitamin deficiencies and a myriad of other causes, pretty much anything.
The same medication is utilized as in traumatic brain injury: amphetamines. They are quite effective in both ADHD and TBI, and as been in active use for nearly a 100 years. Both can be often diagnosed by the same psychometric tests, and can be pretty much indistinguishable.
Of course, finding mechanisms to cope with impaired executive function can help reduce the need for medication later in life.
If you can find the underlying condition, and it happens to be reversible, as is often the case in case of vitamin/mineral deficiencies or some sleep disorders, it can be often eliminated entirely.
However, due to paranoia around stimulants in our society there is some laymen hysteria about giving essentially lifelong access to stimulants to adults. It is an extremely harmful attitude, as medication is nothing but just a tool.