During a quiet moment in the pub over Christmas, a friend of mine leaned over and whispered in my ear. "Do you know where I can get my hands on some smart pills?"

Like me, the friend - who works for an engineering firm - had heard the rumours about modafinil, one of a powerful new breed of brain-boosting drugs that enhance concentration, memory and mental aptitude. Medicines which were designed to treat brain injury, dementia and ADHD are now becoming popular pick-me ups. Businessmen take them to beat jet lag, academics pop them to sharpen their minds and shift workers take them to stay alert.

Online you can find web chat rooms full of glowing testimonies from users, but as the popularity of smart pills spirals, experts are warning that, with no real knowledge of long-term health risks, users are making themselves guinea pigs. In a recent commentary in the journal Nature, Professor Barbara Sahakian and Dr Sharon Morein-Zamir called for a national debate on "cognitive enhancers" and their potential impact on society. Are smart pills "unfair", they ask, and should we do anything to regulate them?

To address their questions, we should begin by looking at currently available cognitive enhancers. Perhaps the nearest thing we have to the ideal "smart pill" is modafinil, a drug designed to treat narcolepsy, which is reportedly being used by businessmen and others to overcome tiredness.

"We know that a number of our scientific colleagues in the US and the UK already use modafinil to counteract the effects of jetlag, to enhance productivity or mental energy, or to deal with demanding and important intellectual challenges" says Sahakian.

Recent paparazzi shots revealed a packet of a brand of modafinil - Provigil - in Britney Spears's handbag.

Although there are side effects - headaches and nausea - they appear to be mild. One survey found that as many as 10% of students at American universities are using Ritalin and Adderall - ADHD drugs - to improve their performance.

Ritalin helps hyperactive children to focus on one thing at a time, but when used in otherwise healthy adults it makes them feel more alert and full of energy - unsurprising when you find out that the active ingredient is amphetamine. However, its side effects include sleeplessness, loss of appetite and in rare cases, hallucinations.

Other so-called smart pills include Aricept (donepezil), an Alzheimer's drug, which studies found could improve the memory of fighter pilots learning new moves on a flight simulator. Finally and potentially the most powerful of all are ampakines, a new untested class of drug designed to boost memory and improve alertness, without giving users the dreaded caffeine shakes.

Does that sound tempting? Most of us can imagine a situation at work where we would benefit from a little cognitive enhancement. And is that really such a crime?

"People have all sorts of natural advantages - some are cleverer, stronger or more beautiful than others," says Michael Gazzaniga, president of the Cognitive Neuroscience Institute in America. "If we can boost our abilities to make up for the ones Mother Nature didn't give us, what is wrong with that?"

After all, as the Nature authors point out: "Most readers would not consider that having a double shot of espresso or a soft drink containing caffeine would confer an unfair advantage at work. So does it matter if it is delivered as a pill or a drink?"

Indeed, they argue that there are situations in which the use of drugs to improve concentration or planning "may be tolerated, if not encouraged", such as by "air-traffic controllers, surgeons and nurses who work long shifts, airport-security screeners, or by soldiers in active combat".

However, they are quick to point out the flipside. "Universities may have to decide whether to ban drug use altogether, or to tolerate it in some situations - whether to enable all-night study sessions or to boost alertness during lectures," they say.

Their biggest concern is for children. The long-term effects of many cognitive enhancers on the developing brains is still unknown.

So what form should any regulation take? Should they only be available on prescription?

"We believe it would be difficult to stop the spread in use of cognitive enhancers given a global market in pharmaceuticals with increasingly easy online access. They call on scientists, doctors and policy-makers to "provide easy access to information about the advantages and dangers of using cognitive-enhancing drugs and set out clear guide-lines for their future use."

A softer approach is possible, they say, because at present, cognitive enhancers are "relatively safe" and yield "only moderate effects".

But, with increasingly sophisticated and even genetically-tailored treatments, truly smart drugs with dangerous large effects on cognition will become feasible.

"Fears have been raised of an overworked 24/7 society pushed to the limits of human endurance, or of direct and indirect coercion into taking such drugs,"

they say.

"If other colleagues at work, or children at your child's school, are taking cognitive-enhancing drugs, will you feel pressure to take them yourself?"

For many, this is no longer a theoretical question.