Students have been warned against taking a so-called "smart drug" to help them stay awake.
There is rising concern that young people are using the stimulant modafinil, which is usually prescribed for narcolepsy, to help them study for longer.
An investigation by Sky News suggested the problem was rife at top universities including Oxford and Cambridge, where students sell the pills to each other for around £2.
Modafinil is a mood-brightening and memory-enhancing psychostimulant, used in the treatment of hypersomnia and narcolepsy
Health Secretary Jeremy Hunt told the broadcaster: "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."
A Department of Health spokeswoman said: "Prescription medicines should only be taken by the person they were prescribed for and in the way their doctor intended
"People may be potentially risking their health if they take medicines intended for other people."