Can a small piece of plastic and copper ward off the harmful effects of computers and mobiles? The stars think so.

It's either a revolutionary device that can deflect harmful radiation - or an ugly pendant worn by gullible celebrities who've got more than 100 quid to throw away. Stars including Madonna and Anthea Turner reportedly swear by theirs; the society magazine Tatler has declared that it "can't get through August without one"; and Jerry Hall apparently likes hers so much that she's bought 10 of the things.

They've all fallen for a piece of plastic, about an inch square, with a copper coil embedded in it and a scientific looking diagram in the centre. If you believe the hype, and 120,000 wearers all over the world appear to. It can cure dizziness, poor circulation, headaches and forgetfulness. Soo Chin, the head of research and development for QLink, claims that the copper coil and a resonating cell inside the pendant vibrate in harmony with the human body, working like a tuning fork to strengthen the body's electromagnetic field. She says this protects wearers from radiation emitted by computers, mobile phones and microwaves. You've got to admit this sounds pretty unlikely, but she insists that the research to back it up is "in progress".

Dr Robert Young, 48, from Utah, is a doctor of science with a PhD in nutrition, and has been paid by the manufacturers to research the pendant's properties. He is convinced that the QLink can help prevent cellular mutations that leave you prone to viruses and infections, and has conducted experiments to show that the blood actually changes after wearing the pendant. Dr Young believes it can "slow down the aging process" and give you a great sex life into the bargain. "When you bring your body into balance, achievements such as exercise, sexual activity and overall well-being are enhanced," he says.

I wore the QLink for a week and it made no difference. I went to bed tired; I woke up tired. So I couldn't recommend it to anyone. But there are people who wouldn't be without theirs. Jeremy Guscott, 35, the England rugby international who is now a TV presenter, has been wearing a QLink for 10 months. Now he sleeps with it, bathes with it, and says the results are incredible. "I felt much better after about a week," he reports. "I'm on the computer quite a lot, so I used to go to bed with tired eyes and find it a struggle to get up. With QLink, that's gone: I wake up more refreshed, ready to go training, and I don't feel tired at all. Generally, I do feel like I have more energy - and I have nothing to gain for saying this. It's given me a healthier feeling. Right from the moment I open my eyes, it's rock and roll."