However, he also noticed that when he used Vyvanse, a prescription drug to treat binge-eating and attention deficit hyperactivity disorder (ADHD) in children and adults, he could “hyper-focus” on his work. Now the 30-year-old (who chose a pseudonym when interviewed for this article) “pops a small dose” every time he needs to power through his work as an analyst for an investment group in the US.

He does not have a prescription for the drug but buys it by the pill from a friend who does. Mr Smith has also sold pills to friends who feel the need for help to concentrate or work long hours. This is never for profit, he is keen to point out, and often puts him out of pocket. “I’m not a professional drug dealer,” he says. “I simply help out friends when they request it.”

Such stories do not surprise Michael Sinclair, apsychologist based in the City of London. He is used to seeing a steady trickle of younger clients who report taking drugs such as Vyvanse or Adderall that are prescribed for ADHD, or Modafinil, which is used to treat narcolepsy, in an attempt to help people concentrate or work for longer periods. “People are buying it off each other and on the internet.”

Patrick Curtis, a former banking analyst and founder ofWall Street Oasis, a networking and career site for those working in finance, observes a generational shift in the use of drugs. There is a move from alcohol and cocaine to so-called “performance-enhancing drugs” among younger Wall Street workers. “The demands of the job have increased and the drugs of choice have evolved as a result,” he says. He believes the pressures and competition are harder for this generation. “Students 20 years ago barely even knew what investment banking was in their freshman and sophomore years. Today, they know how to build leveraged buyouts, run a discounted cash flow and explain complex derivatives.”

One technology sector worker, who prefers to remain anonymous, took Adderall to help him get through his high-school exams. “When you have 36 hours’ worth of exams in the space of three weeks there simply isn’t a way to get prepared for everything ahead of time. Adderall, whether [its effect] is physiological or psychological, helps you get through that.”

A downside was that it would sometimes make him focus on the wrong thing: “You start reading on a certain subject, not related to your course, and you keep reading and researching it because you’re interested in it.”

The numbers of people who use such drugs for productivity reasons rather than medical ones is hard to gauge. One report in the US estimated 34 per cent of college students had used ADHD medication primarily in periods of high academic stress. Others have suggested the figure is lower. Researchers at King’s College London and the London School of Economics found that 9 per cent of surveyed students had used Modafinil, Adderall or Ritalin at least once. Moda¬finil is approved for helping shift workers stay alert in the US. But as a 2012 re¬p¬ort, called “Human enhancement and the future of work”, pointed out, “the definition of shift work could encompass many different work patterns”.

Johann Hari, a former Independent journalist who left the newspaper after admitting plagiarism and then wrote a book about the war on drugs, recently told of his use of the anti-narcolepsy drug Provigil. “You can do even more work, and be constantly processing information, and sleep only four hours a night.” He believes he became addicted. The long-term effects of such drugs on healthy minds is yet to be determined.

Michael Craig is a consultant psychiatrist at the London-based Nightingale and Maudsley hospitals and works with adults with ADHD. He has encountered patients with no medical need demanding pills. One mother of a teenager preparing to study in the US was particularly aggressive in her demands for a prescription. “She felt that it wasn’t a level playing field if her child was not using them.”

Many of his patients are misdiagnosing themselves, believing their inability to focus for long stretches is a physiological defect. ADHD is unusual in that “people want to have it”, says Dr Craig. “You don’t get that with bipolar or schizophrenia — people will ask for their money back in a private clinic or complain if it’s in the public sector if you don’t diagnose them with adult ADHD.”

This resonates with Dr Sinclair who fears that people are trying to use drugs to “eliminate the worst aspects of themselves”. Those who come through his door tend to be perfectionists “who fear failure”. He is concerned that people are not allowing themselves to feel tired or down. “Actually, it’s OK to feel low”.

John Harris, a bioethics professor at the University of Manchester, is more sanguine. He is broadly in favour of technology and drugs that improve performance. “If everyone’s working all hours the pressure is on.” Just as people use coffee and alcohol, so prescription drugs might be a rational response to pressure, he feels. “We should operate on the pressure not the drugs.”

Ben Willmott, head of public policy at CIPD, a UK human resources body, agrees that the pressure must be tackled. “The best way for employers to reduce the risk of employees using this type of drug is to ensure that line managers are trained effectively, working hours are appropriate and that people are not put under excessive and prolonged pressure.”

As for the pseudonymous Mr Smith, he shrugs off the notion that he is under too much pressure. “Honestly? I love my job and wish I could do even more work.”