Smooth and sophisticated, confident and cosmopolitan, the image enjoyed by coffee has always been a subtle blend of exotic provenance and urban cool, the socially acceptable stimulant with the seductive aroma.

In every town and city high street, coffee bars are springing up faster than you can say “tall latte to go”. And in staffrooms across the land, fatigued teachers in need of a boost at morning break reach for a shot (usually instant) of caffeine in a cup. After all, in the age of GM food and junk food, cholesterol and mad cows, what is there to worry about in a cup of coffee? Well, consider this. Caffeine - the most active ingredient in a cup of coffee - is the most frequently and widely consumed drug in the world - and new research suggests that its effects might extend beyond those of the benign pick-me-up we rely upon to get us going in the morning. Millions of us do - 80 per cent of people in the UK enjoy a brew of the roasted beans, and every person in the world drinks an average of one cup a day.

Peter Rogers, a psychologist at the University of Bristol, has researched the effects of caffeine. In his tests, volunteers drank fruit juice with or without caffeine. They were then given simple tasks to perform and asked to record their mood. The results surprised him. Moderate coffee consumers - those usually drinking two or three cups a day - who took the placebo juice were more tired, angry and dejected than the non-consumers.

“We were surprised,” he says. “Caffeine consumers are not really benefiting from it - they are just reversing the negative effects. You go without caffeine overnight, consume it again in the morning and that puts you back to normal levels of alertness - reinforcing your liking for the drink.”

Mr Rogers, a tea drinker, says that while people are aware of the psycho-stimulant effects of coffee and use it accordingly - drinking it in the morning to give them a lift, but not in the evening in case it keeps them from sleeping - they do not think of it as a drug. We are born with a natural aversion to bitter tastes and, he believes, we develop a taste for coffee by association with its stimulating properties. This then develops into a mild form of addiction. Deny coffee drinkers their caffeine and they show all the classic symptoms of withdrawal - headaches, irritability and fatigue. Give them their caffeine and they return to normal. But, he adds:

“If we didn’t drink the stuff in the first place, we would be just as alert and we wouldn’t experience early morning fatigue. I’m pretty convinced that most of the effects are withdrawal reversal effects - although plenty of scientists disagree.” Caffeine can save your life because it can stop you falling asleep at the wheel, but the reason you are falling asleep is because you are going into caffeine withdrawal.

This “negatively reinforced liking” helps to explain the popularity of caffeine-based drinks. “Most people who consume it are not greatly harmed by it, and the withdrawal effects are relatively minor,” says Mr Rogers. “Although the impact of caffeine on the individual is small, because so many people drink it worldwide, those effects make it an important drug.”

Although tea, hot chocolate and cola drinks also contain caffeine, it is coffee drinkers who receive the most concentrated dose - about 70mg per cup, compared to about 50mg for tea and 20mg for hot chocolate. And one of the reasons so many cold and flu remedies contain caffeine, Mr Rogers suggests, is not that it helps sufferers get better, but that it stops them feeling worse by keeping the withdrawal effects at bay.

All manner of claims - positive and negative - are made for caffeine. On the plus side, it has been associated with reduced incidence of gallstones, hayfever, asthma, Parkinson’s disease and colon cancer. It has even been suggested that regular consumption reduces the risk of heart disease, although this might be explained by the predominance of coffee drinking among the middle classes, who have a healthier diet and lifestyle than lower socio-economic groups. Against this, other studies have shown that coffee’s diuretic and laxative effects can harm the body’s ability to absorb valuable minerals such as calcium, iron and zinc. Heavy caffeine consumption has been linked to increased cholesterol levels, miscarriages, ulcers and arthritis.

Caffeine’s stimulant properties are undisputed. Immediately after having a cup of coffee, our pulse rate and blood pressure go up, the supply of adrenalin to the brain increases, making us feel more alert, and stress hormones are released into the bloodstream. But this high is followed by a low, setting up a cycle of dependence that can be relieved only by the next cup.

Helen, an art teacher from south London, knows the feeling. She gave up her 10-cups-a-day habit last April after her dentist told her it was staining her teeth and because she thought it was exacerbating her eczema. “When I first gave up I had the most horrific headaches, like migraines, and a weird spacey feeling all the time. The feelings of withdrawal lasted four or five days. After I got through those, I found I had a lot more energy. Before, when I was drinking coffee, I would get a sort of bad-tempered giddy feeling. After I gave up I was calmer, a lot more in control.

“I started drinking it again though, a little bit here and there. It sort of crept up on me. I had a craving and somebody bought me a cappuccino. I don’t drink alcohol and I don’t smoke, so coffee is my only stimulant. I love it. I like it strong and milky. I drink about eight cups a day now - but half of those are decaffeinated.” Although she is not worried about her coffee drinking she is sure it’s an addiction. “I couldn’t leave home in the morning without having a cup. You have a coffee to make you feel up, but it doesn’t really - it just makes you feel normal.”

Dr Adam Carey, of the London Centre for Nutritional Medicine, says coffee mimics the adrenalin rush we experience when we are in danger. “It stimulates all the same receptors and blood flow as if we were getting ready to run away or have a fight. And that’s exactly why people use it - to pick themselves up, give them a bit of a kick, to get them going. But if you continually rely on caffeine for your drive, just like any drug, you become habituated to it. You find it difficult to get up in the morning and do things unless you have had a cup of coffee.

“If you take more than two or three cups a day, every day, and if you think, ‘I couldn’t get through the day without a cup of coffee’, you should be concerned. Like all these things, there are degrees. People really need to ask themselves, ‘Do I need a coffee fix?’ If the answer is, ‘If I don’t, I won’t feel good’, you have a problem.”

CALL OF THE CAFFEINE

* The first coffee to arrive in Europe was sold in pharmacies in the 17th century as a medicinal remedy and was known as Arabian wine.

* Bach wrote a coffee cantata in 1732 and the French writer Voltaire reputedly drank 50 cups a day.

* Cappuccino is so called because the peak of whipped milk resembles the hood of a Capuchin monk’s habit.

* In 1997, Britons drank 1.76kg of coffee per head, putting us 15th in the European league of coffee drinkers. Scandinavian countries, the Netherlands and Belgium drank the most while Italy, the home of coffee culture, was 11th.

* On average, men drink slightly more coffee than women, two-thirds of us take milk and the same proportion add sugar.

* Filter and Turkish-style coffees have the highest concentrations of caffeine, while a single espresso or instant coffee are lowest.

* Caffeinism is a recognised medical condition. In 1997, a British army officer accused of false accounting walked free after a court martial heard he suffered from the condition and drank up to 15 pints of tea a day.