Medication for the treatment of attention-deficit hyperactivity disorder (ADHD) is in widespread use globally. There is considerable data suggesting that overall, the adverse effect burden from this use is dose dependent and is in the mild to moderate category, but few comprehensive reviews exist of the epidemiology of adverse effects alone. This review provides a general and systems-specific summary of the scientific literature regarding adverse effect data for the drugs in general use for the treatment of ADHD. Although several areas lack definitive data, current evidence suggests that, for the majority of those treated for ADHD, the medications currently available pose little in the way of risk of significant harm. Epidemiological data suggest a low incidence of serious adverse effects, whilst the less serious adverse effects, such as insomnia and anorexia, are relatively common. Also, some specific areas of study suggest lower risks of harm than previously thought, e.g. tic disorders and seizures. However, pre-existing conditions and other interindividual differences may raise the risk of harmful adverse effects, which adds emphasis to the need for careful pretreatment assessment and monitoring. Potential but unlikely long-term treatment effects need to be investigated as carefully as possible, particularly with regard to cardiac sequelae and carcinogenesis. There are both overlaps and differences between the adverse effects of stimulants and nonstimulants, such as atomoxetine. For example, the latter shares the stimulant group's potential for changing cardiovascular parameters, but may not cause insomnia.