Here's a thoroughly modern ethical dilemma to chew over. You go for a job interview but are pipped to the post by another person who seems wholly underwhelming and is less well qualified. The feedback is that she came across as more zappy and focused. Over coffee, she shares that she's popped a tab of Ritalin before the interview. How does that make you feel?

Or here's another one. Senior members of the academic department that you work in share their tip for being able to fly over to the US and deliver a lecture the same day while looking fresh and sounding perky. They regularly take the drug Modafinil to counter jet lag and enhance their cognitive function. You're not keen, but as a junior member of the team, you want to perform as well as possible. So do you try to get your GP to prescribe some or, like most of your workmates, buy some on line?

These challenges aren't theoretical but real. Cognitive-enhancing drugs, also known as "smart" drugs are already being used to help people with Alzheimer's disease, schizophrenia, attention deficit hyperactivity disorder (ADHD) and brain injury. But is there burgeoning use among university students and others wishing to boost their brain power? It is this that's posing the ethical dilemmas.

Barbara Sahakian, professor of clinical neuropsychology at Cambridge University, described how the smart drugs fit in to our quest to keep our minds functioning longer and better in our ageing population, in a lecture last night at the Royal Institution, organised by Prospect magazine.

The three drugs in common use are methylphenidate (Ritalin), modafinil (Provigil) and atomoxetine (Strattera). All boost neurotransmitter chemicals (noradrenaline, acetylcholine, dopamine) at the junction between nerves to improve transmission or electrical activity.

Even modest improvements in cognitive function can make a big difference to people's lives. It can make the difference between someone with schizophrenia being able to live independently or not. Or a kid with ADHD being able to stop his impulsive behaviour and stay out of trouble at school. And there are huge financial implications in being able to reduce the cost of long-term care for people with Alzheimer's if a drug can slow their cognitive decline by even 1%.

Professor Sahakian said that the smart drugs are just one approach to boosting cognitive function. Exercise (three brisk walks a week will do, she said) and learning new things throughout life are most important. And connecting with others, giving of yourself and maintaining your curiosity about life are the other cornerstones of mental wellbeing.

She warns against the potential dangers of people under 20, whose brains are still developing, taking smart drugs like Ritalin unless they have a condition such as ADHD. But an estimated 16-20% of US college students take smart drugs and there is reason to believe its spreading to the UK.

The field of neuroethics is developing to look at challenges posed by advances in the neurosciences. Some arguments for smart drugs include removing disparity in society and boosting performance. In a lengthy operation, it is best to have one surgeon do the whole job and a tab of Modafinil may be better than caffeine which, in high doses, can cause tremor.

The US army is interested in smart drugs to allow troops to complete military operations as quickly as possible. But the potential harms include the fact that we don't know about long-term effects, especially in developing brains. People may feel coerced into taking them. There's a potential for abuse and you may get unwanted effects like persistent memories. Is it cheating to take the drugs? Will it turn us into mechanistic beings who don't feel we've earned the right to feel proud of our achievements? Will working hard and being motivated become antiquated concepts? Will we become a homogenous society and will we lose creativity?

The last concern is practical. There is no licence for prescribing smart drugs unless you have a specified condition. No self-respecting GP is likely to prescribe it. And buying drugs online is fraught with dangers. You have to check you have no contraindications, that it won't interact with any other medication and that what you get in the post is what it says on the box.