Can I first say that I am not an authority on attention deficite hyperactivity disorder (ADHD) and I can find no definitive guidelines concerning this condition and scuba diving and no doctor can give you a professional opinion on a website.
So can I just discuss basic principles?
Certain medical conditions are an absolute contra-indication to diving, brittle diabetes and refractory asthma being two that spring to mind. This is because the condition itself could lead to the death of the sufferer during a dive. In addition it could put the diver's buddy at risk in any rescue attempt, or if he needs help you may be unable to provide it because of your condition.
The second and equally important consideration is the effect of medication on your ability to function underwater. High partial pressures do not affect the bioavailability of drugs but do affect the body's response to some of them.
You are clearly unfit to dive if your condition, or its medication, make you a risk to yourself or others.
Amphetamine is a stimulant and therefore affects a divers decision-making processes. There is also a powerful theoretical risk of amphetamine making you more likely to suffer a fit due to the synergistic effects of the drug and the the well-documented effects of high partial pressures of both oxygen and carbon dioxide.
So, Alex, I strongly recommend you consult a local expert in diving medicine and take his advice.