"Certain brain chemicals, called neurotransmitters, are known to play a role. Among others, dopamine and norepinephrine are two key neurotransmitters that, when present at the correct, balanced levels in the brain, contribute towards maintaining alertness, increasing focus, and sustaining thought, effort, and motivation. Some of today’s medications to treat ADHD are used to increase the levels of these brain chemicals in order to improve the symptoms of ADHD.
So now let’s look at how these medications work. Although the modes of action of methylphenidate and amphetamines are not fully understood, they are thought to act to increase availability of norepinephrine and dopamine within the brain.
When these medications work effectively, they help to increase attention and decrease impulsivity and hyperactivity in patients with ADHD.
Methylphenidate and amphetamines both work by increasing levels of norepinephrine and dopamine within the brain. 
Here’s an interesting fact!
Methylphenidate and amphetamines are referred to as ‘stimulants.’ However, these medications may also be referred to as ‘dopamine and norepinephrine reuptake inhibitors and releasers’—based on how they are thought to work in the brain!
Besides amphetamines and methylphenidate, there are other medications that also serve to treat ADHD, but in slightly different ways. Atomoxetine works to increase levels of norepinephrine, and guanfacine is an alpha-2A-adrenergic receptor agonist whose mechanism of action is not known."