Aims
The use of substances for cognitive enhancement has become a relatively well-studied phenomenon in recent years. However, few studies deal with the negative and unintended consequences of such practices. This article uses two data sets to explore and discuss the doubt and negative consequences that affect people using substances in the pursuit of enhancing cognition.

Methods
Data for the study are drawn from an online discussion forum on substances for enhancement and from ethnographic fieldwork carried out among university students in New York City. Taking a quali-quantitative approach, we combine digital text analytic tools with qualitative analysis and readings.

Findings
Using prescription stimulants and other substances for cognitive enhancement generates considerable uncertainty in terms of unclear effects, varying practices and ambivalent ethics. While the negative effects are not something easily discussed in person, references to them are very common in the online discussion forum.

Conclusions
People who use substances for enhancement have developed a ‘folk pharmacology’ that seems to play an important role in how they perceive the negative effects. This may make people who engage in these kinds of enhancement practices less able to make informed choices about their use of these substances.