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Adderall, Ritalin and other “smart drugs” have become popular among college students and young professionals, who use them to enhance performance. The drugs are normally prescribed to treat attention deficit hyperactivity disorder (ADHD), but healthy students use them to get a leg up in school, by improving focus, concentration and memory. The question is, do they work?

Maybe not, according to a new study from the University of Pennsylvania. Students who took Adderall didn’t actually perform better on tests of cognitive function — they only thought they did. Casey Schwartz blogged about the findings on the Daily Beast:

The research team tested 47 subjects, all in their twenties, all without a diagnosis of ADHD, on a variety of cognitive functions, from working memory — how much information they could keep in mind and manipulate — to raw intelligence, to memories for specific events and faces. Each subject was tested both while on Adderall and on a placebo; in each condition, the subjects didn’t know which kind of pill they were receiving. The researchers did come up with one significant finding. The last question they asked their subjects was: “How and how much did the pill influence your performance on today’s tests?” Those subjects who had been given Adderall were significantly more likely to report that the pill had caused them to do a better job on the tasks they’d been given, even though their performance did not show an improvement over that of those who had taken the placebo.

It’s not surprising that Adderall gave students an inflated sense of productivity, Schwartz writes, given that the drug — a close cousin of amphetamine — “unleashes the feel-good neurotransmitter dopamine, triggers the brain’s reward system, and can produce a mild sense of euphoria.” (More on Time.com: Drug Surprise: Meth Makes You Feel Almost As Cuddly as Ecstasy)

So whether or not the drug boosts performance on cognitive tests in the short-term, could it be possible that its “euphoric” effect simply makes studying more pleasurable, helping student achievement by ramping up enthusiasm for academics overall? (More on Time.com: Clues to the Genetic Roots of ADHD)

Schwartz points to a personal essay about performance enhancement by a recent college senior, Molly Young. Writing for N+1, Young noted, “Of course, I could have studied in college without Adderall, just like I did in high school — I just couldn’t have studied with such ecstasy.”

Then again, ecstasy doesn’t necessarily mean creativity, which is another marker of cognitive performance, and one that’s hard to pin down in a scientific study. “Though I could put more words to the page per hour on Adderall, I had a nagging suspicion that I was thinking with blinders on,” wrote Slate’s Joshua Foer in 2005. (More on Time.com: A Five-Minute Brain Scan Tracks Kids’ Development and May Spot Disorders)

UPDATE: It bears noting that the new study, which has not yet been published (it was presented at the annual Society of Neuroscience conference in November), is contradicted by a body of evidence showing actual cognitive benefits of the drug. Healthland’s Maia Szalavitz reported:

The benefits of enhancement include increased alertness and focus and improvement in some types of memory. Research shows that in normal people, stimulants consistently and significantly improve learning of material that must be recalled days later — exactly what you want from a drug when you are prepping for exams. The drugs even seem to improve certain aspects of judgment. One study of 36 normal women and men found that they were more likely to choose to delay gratification and receive a larger monetary reward when given amphetamines than settle for a smaller amount of money immediately. Improvements in memory and cognitive control have been reported in multiple studies, mainly using Ritalin and amphetamines.

Research suggests, however, that the drug doesn’t improve performance evenly. Many users receive no performance boost, as evidenced by the current University of Pennsylvania study as well as previous work. Szalavitz wrote:

Interestingly, those who have the least ability in a particular area are likely to see the greatest drug-related improvement. In fact, on some tests of cognition, the smartest people actually showed performance reductions, a result that may address some of the concerns over “cheating”: on tasks involving working memory and impulsivity, stimulants had a leveling effect, allowing below-average performers to catch up to their peers, not dominate them. According to Farah, the typical student user is actually not the overachieving brainiac but a “white male frat brother with a B average.”

Why it works for some and not others isn’t entirely clear, but nota bene: don’t take Adderall or Ritalin without a doctor’s prescription, especially if you suffer from psychosis, mood disorders or high blood pressure.

This post has been updated to reflect the fact that past research suggests taking “smart drugs” may have a significant effect on performance.

Related Links:

ADHD: A Global Epidemic or Just a Bunch of Fidgety Kids?

Training Your Brain to Learn Better (Even Without Drugs)

They’re Baaaack! How to Avoid a Holiday Clash When the Kids Come Home From College