As a mental health professional, I will add the following to the comments made by Lloyd (who is an eye doctor):
The decision on diving with ADHD, medicated or not, is made on a case by case basis. An individual with this disorder would need formal medical clearance in order to proceed with scuba training of any sort.
The following introduction to diving with a mental/behavioral disorder written by Dr. Ern Campbell (aqua Scubadoc) provides a good orientation to the issues:
"Little research has been done to factualize the relationship between mental conditions and scuba diving. Other than the obvious proscriptions against someone diving that is out of touch with reality, severely depressed and suicidal or paranoid with delusions and hallucinations---one has to consider the many who can dive with everyday anxieties, fears and neuroses.
Successful divers have a profile that is positively correlated to intelligence, is characterized by a level of neuroticism that is average or below average, and score well on studies of self-sufficiency and emotional stability.
There are some actual psychological disturbances that are well known to all but which are poorly studied and documented as concerns the risks of scuba diving. These include the depressions, bipolar disorder, anxiety and phobic states, panic disorders, narcolepsy and schizophrenia.
In addition to the risks caused by the condition itself, one must add the possible hazards of effects and side effects of medications - either as taken singly or even more dangerous, in combination. Needless to say, there have been and probably will not be good scientific studies that will indicate the safety or danger of any given set of conditions and drugs. The role of medication in diving is usually less important than the condition for which the medication is being used. A mood-altering medication is plainly powerful and should be used with care in diving. Drugs that carry warnings as dangerous for use while driving or using hazardous equipment should also be thought of as dangerous for divers. The interaction between the physiological effects of diving and the pharmacological effects of medications is usually an educated supposition. Each situation will have to be carefully evaluated individually, and there is no general rule that applies to all.
Finally, every diver has his own personality makeup, which may respond differently to abnormal physiological states and changes in the environment from the effects of various gases under pressure. Such states as inert gas narcosis, carbon dioxide toxicity, oxygen toxicity, HPNS, deep water blackout all can cause reactions that are similar to a psychoneurotic reaction or one of the organic cerebral syndromes. Therefore the diver, the dive instructor and the certifying physician all must be aware of the all the possibilities and protean manifestations of each and every individual case before allowing or disallowing diving with psychological problems."
All divers must show attention, concentration, impulse control & behavioral modulation that are fully adequate to meet the demands of safe scuba, and this obviously is a concern with those diagnosed & treated for ADHD.
In addition, there are unanswered questions regarding the possible effects of stimulant drugs such as methylphenidate. High among them is what effect they may have on the development of oxygen toxicity secondary to the elevated partial pressures of 02 that are an inherent part of breathing compressed air, and more so nitrox, at depth.
This is educational only and does not constitute or imply a doctor-patient relationship. It is not medical advice to you or any other individual, and should not be construed as such.
Best regards.
DocVikingo