Sir

Henry Greely and colleagues identify critical areas of public discussion about perceptions and use of drugs that are alleged or expected to improve cognition, in their Commentary 'Towards responsible use of cognitive-enhancing drugs by the healthy' (Nature 456, 702–705; 2008).

Stimulants and other drugs proposed as potential cognitive enhancers are known to create profound state dependence, a phenomenon in which information or associations learned while 'under the influence' of a drug will later be remembered or used only when the learner has again taken the drug. Thus, individuals who use amphetamines to improve their learning of new information may indeed learn slightly faster or with less effort than those who do not use such drugs. Later, however, they may not remember or use the learned information unless they take amphetamines or related drugs again.

How and when state dependence occurs has been studied extensively in humans and other animals, in well-controlled learning tasks in laboratory settings. State dependency of cognitive enhancers would dramatically influence drug use, the permanence of learning and the ability to use information in new conditions. Indeed, state dependence can complicate clinical use of pharmacotherapies for such disorders as anxiety and attention-deficit hyperactivity disorder.

Greely and colleagues note critical research and policy questions that societies must consider to shape expectations about putative cognitive enhancers. We urge that the discussion includes the known and profound state-dependent effects of these agents.

See also Risks and benefits may turn out to be finely balanced Much ado about cognitive enhancement A medical view of potential adverse effects Patterns of drug use have varied throughout history Careful use helps me do better research, and society benefits Enhancement means a broader role for physicians