In this kind of drug-taking, sensation isn't the goal but the effect of pursuing other goals. Recognising it as a distinct form of drug use – for neither medication nor recreation, but for application – raises several questions: one is whether these drugs are effective over sustained periods (the 2010 review of work on modafinil and methylphenidate found only two studies for each drug that looked at the effects of repeated doses, and the longest of those lasted just six weeks), and another is what effects they might have on their users' health. Sahakian emphasises the need for a long-term study "to determine whether these cognitive-enhancing drugs are safe for healthy people to use," adding that "our brains are in development into late adolescence and even young adulthood, [so] these safety concerns are particularly great for young, healthy people."