Your Editorial 'Defining natural' (Nature 452, 665–666; 2008) indicates that cognitive-enhancing drugs have only mild effects similar to caffeine, and the News story 'Poll results: look who's doping' (Nature 452, 674–675; 2008) highlights the general increase in their availability through Internet purchase and possibly diversion of prescriptions. By far the most frequently prescribed of these drugs — the stimulant medications amphetamine and methylphenidate — have abuse potential and therefore warrant particular concern.

The World Health Organization (WHO) tracks by country the yearly production, supply and consumption of psychotropic substances, including stimulant drugs. The most recent figures suggest that we should evaluate carefully whether diversion of methylphenidate and amphetamine may be increasing in the United States. From 1995 to 2006, consumption estimates by the WHO and prescriptions recorded by the healthcare-information company Verispan both increased linearly. Moreover, the 268.9% relative increase in the WHO estimates of consumption, from 4.66 to 17.9 defined daily doses per 1,000 population, far exceeded the relative increase in number of prescriptions per year, from 15,044,359 to 30,137,136 (100.3%).

The greater increase in consumption could be related to a shift in the age of individuals treated for attention-deficit hyperactivity disorder, as more adults are now seeking diagnosis and treatment. This could influence debate over whether some prescriptions for clinical treatment of adults are being appropriated instead for performance enhancement or recreational purposes. History teaches us that either of these could escalate into misuse.

We have serious concerns about the dramatic increase in national consumption and the smaller, but still large, increase in annual prescriptions of stimulant medications. These rises should make us vigilant to avoid a potential repeat of past episodes of abuse.