ADHD brains require more dopamine and norepi to point the neurons in the right direction at the right times. You can add those in a number of ways that all have different side effect profiles and different likelihoods of worsening the root problem / causing dependence, or even ultimately destabilizing the disorder in the long run entirely.

Stimulants in general (caffeine, nicotine, cocaine, amphetamines) will all boost the chemicals an ADHD brain needs to focus right the hell now. They also have to be used super carefully because they can cause hypertension, heart problems, dependence, and psychosis. Cocaine and nicotine in particular also cause extra heart problems and in the case of nicotine Cancer.

A lot of people also take them too close to bedtime because it's the only way they know how to hold still long enough to fall asleep, but because they're still stimulants they fuck up the rest of the circadian rhythm once the person is asleep and greatly worsen the underlying ADHD (if you have this problem ask your doctor what OTHER / nonstimulant medications you can take for ADHD at night because there are better options).

Personally I don't even take my non stimulant SNRI Strattera at night because while it's not acting directly on norepi it IS acting on the reuptake inhibitor, so I take clonidine at night.