Students at some of Britain's best universities are potentially putting their health at risk by using a 'smart drug' bought off the internet, Sky News has been told.
Doctors have warned that increasing numbers of teenagers are using the prescription-only medicine modafinil to stay awake and alert for long periods of time.
The drug is designed to combat the sleeping disorder narcolepsy.
However, research shows it is available to anyone from dozens of online retailers.
Anecdotal evidence suggests there is a black market at universities, including Oxford and Cambridge, with students selling it to each other for around £2 a pill.
Some students are now said to be demanding drug tests before exams to stamp out a practice they believe to be equivalent to cheating.
Some students are turning to the internet to get the drug One Oxford student told Sky News that he believed up to a quarter of his student friends had taken modafinil.
But academics say that the long-term effects are unknown, and medicines' watchdog the Medicines and Healthcare Products Regulatory Agency says the drug should not be taken without a prescription.
Barbara Sahakian, professor of clinical neuropsychology at Cambridge University, said there had been an increase in the number of students taking the drug in recent years.
"A lot of young people are purchasing these drugs over the internet.
"[It] is a very unsafe way to get these drugs because you don't really know what you're getting and you don't know if it's safe for you as an individual."
She said some students feel peer pressure to take the drug in order to keep up with their fellow students.
There can be pressure to take modafinil, says Professor Barbara Sahakian "There's this kind of coercion that goes on.
"I think a lot of students feel it's very unfair that other students are taking these drugs during exams and they feel that they're losing out because the other person has a competitive advantage.
"Some students feel when you go into the exam perhaps there should be a test to see whether you're on the drug or not, because otherwise there's no way of detecting whether you are or not."
Prof Sahakian believes that the increasing use of modafinil raises serious concerns both for students and society.
"What are the effects of putting a drug into and changing the chemicals in your brain as your brain is developing?" she asked.
"What will society be like in the future? Will we all be just popping pills to stay awake, and alert and keep working? Will we accelerate into a 24/7 society? Is that what we really want?"
Health Secretary Jeremy Hunt told Sky News: "I'm very concerned. Let's be clear. This is drug abuse.
"To all those young people we completely understand the pressures of taking exams but you are playing with fire if you take drugs that haven't been prescribed.
"You don't know what the effect will be on your mind and body. It's a very dangerous thing to do and I would strongly encourage students to think again before doing this."
Laurie Pycroft believes around a quarter of his friends have taken the drug Laurie Pycroft, a masters student at Oxford University, admits taking the drug every few weeks.
He told Sky News: "Some people report that they become very focused or very good at concentrating on a repetitive task.
"I have found the ability to go without sleep, when necessary, quite effective. It's essentially like caffeine, just a bit more effective and with less jitters.
"The people I've met who offer me modafinil for sale, they've all been fellow students, or academic types, rather than your stereotypical wheeler-dealer in a hoodie down a dark alley."
Doctor Anders Sandberg, research fellow at Oxford University's future of humanity institute uses modafinil every one or two weeks.
While he believes he is doing himself no harm, he would rather be able to get the drug from his GP.
Oxford University says it had not seen evidence of a modafinil problem "Going via an internet drug store means the money ends up in the grey market, and that's problematic. You might be feeding your money into a market that is actually doing a lot of harm in the world.
"It would be much better if it were in the open market, which would mean that we could actually control that it's actually healthy, that side effects get reported, that you could actually study it properly."
He added: "I don't regard the use itself as immoral or problematic. I'm not competing with anyone else. I am taking the risks on my own."
Buying prescription-only drugs is not illegal, however, supplying them is.
Universities UK says there is no firm evidence to suggest taking 'smart drugs' is widespread among students and called for more research to discover how common it is.
In a statement, it said: "We would be very concerned if the impression were given that most students at UK universities are now taking … 'smart drugs'.
"We are not aware of any new research or data to suggest that such drugs are widely used and available among the UK's higher education student population of 2.5 million students."
It said however, that it would have "grave concerns" about students taking drugs not prescribed to them.
An Oxford University spokesman said: "If 'cognitive enhancement' drugs are a particular problem at Oxford we have yet to see any substantive evidence for it."
In a statement, spokesmen for Oxford and Cambridge universities both said they strongly advised students never to take prescription-only medicines without a doctor's recommendation.
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