At 4pm on a Thursday in central Edinburgh, Aaron Campbell is beating the traffic with an armoured wolf. The shaggy creature leaps across a wooden bridge and through a market-place unimpeded by the assortment of gnomes, dwarves and orcs milling around. No, it's not Lothian Road on a fancy dress club night, but the virtual world of Northrend, a newly released continent in the hugely popular online role-playing game World of Warcraft.

Aaron, 20, and several other members of his playing "guild" have met up at the Ministry of Gaming cafe on Bread Street to try out WoW's hotly anticipated expansion set, Wrath of the Lich King, which was released at midnight on Thursday. Three of them went as the clock struck 12 to pick up their pre-ordered copies at Gamestation on Princes Street. The shop sold out the same night.

The phenomenon was repeated all over Europe, with many fans on Oxford Street in London dressing as their favourite blood-elves or trolls. Eleven million people worldwide play WoW, according to the game's American makers, Blizzard Entertainment, making it the world's most successful MMORPG (massively multiplayer online role-playing game). Players spend hours each week going on quests to kill "bosses" (monsters), often grouping together in a guild and communicating with microphone headsets, with the aim of rising up the levels ("levelling"). For many, the game is a sociable alternative to slumping in front of the TV, but the release of Lich King has led some psychiatrists to raise the alarm about young players' potential to become addicted.

"Some of my clients will discuss playing games for 14, 16 hours a day at times, without breaks and without attending to their physical needs. For those, the consequences are potentially severe," said psychiatrist Dr Richard Graham of the Tavistock Centre in London on Thursday. He spoke of his concerns that such prolonged gaming could produce "a sort of socially withdrawn figure who may be connecting with people in the game but is largely dropping out of education and other social opportunities".

Players at Ministry of Gaming on Thursday agreed there was the potential for certain people to get hooked, but stressed that, played in moderation, World of Warcraft can bring people together.

Stephen Yu is a WoW veteran. The affable 25-year-old software engineer first discovered the game in 2004 and has been playing ever since. His guild meets three nights a week at the Ministry of Gaming and the playing sessions are sometimes followed by a pint. In his view, playing the game with his friends is the best thing about it. Stephen is conscious of managing his playing. "When I first started, it was pretty much all I could think about," he says. "Now, it's a few hours per night, if that; I maintain a balanced life. But there are some people who are easily hooked and play from the moment they get up till the moment they go to bed."

Allan Berry, 22, a journalism student, also plays moderately. He doesn't play computer games on his own, only with his friends at Ministry of Gaming. He agrees with Stephen: the best thing about WoW "is and always will be the social side, working together for a joint goal".

Aaron, who studied interactive media and design at university and now works at the Ministry, plays more than the others. He reckons he can play for eight hours a day.

Blizzard Entertainment, for its part, has built in rewards for players who take breaks and there is also parental control so parents can limit when and for how long the 12+ certificate game may be played. Still, so-called "Warcraft widows" are well represented on online support groups for gamer spouses, like Gamer Widow and WoW Widows, which has nearly 3800 members and a pithy strapline: "Stop levelling and start living!"

Computer games are huge business, with Scottish companies like Rockstar North, which makes Grand Theft Auto, getting their slice of the action. Role-playing games appear to be particularly enthralling. The extent to which real and virtual lives can become entangled was demonstrated yesterday with news that a British couple are divorcing after the wife found her husband's alter ego on Second Life forming a bond with another virtual woman.

When it comes to getting obsessed, some people, it seems, are more likely to develop problems than others. Dr John Charlton, a psychologist at Bolton University, has done research on online gaming and addiction and has found that people with personality traits linked to Asperger's syndrome are more likely to become addicted to the games.

"We found people who tended to be anxious and introverted, personal characteristics which have been seen to be related to low self-esteem, tended to get higher addiction scores," he says. "Those character scores do tend to be linked to Asperger's."

He adds: "We are not saying these games can cause Asperger's: that's nonsense." Rather, that "anxious and introverted people have problems communicating with other people face to face. These games allow them to join in and socialise, but not face to face".

The study concluded that around 3% of the players showed signs of being hooked - that is, missed meals and went without sleep to carry on playing.

"The definition of addiction is something that's doing you harm and you can't stop doing it," he says. "It was stopping them getting their lives together, they were retreating into their bedrooms rather than having proper relationships."

And he recognises that it can be socially beneficial. "By no means can this be said to be all bad: if you find it difficult to make friends face to face, it can be a good thing."

Ewan McMahon, Ministry's manager, says the length of time people play varies greatly and agrees that "moderation is the key". The clientele is varied. "We get businessmen in suits," he says. "Where 20 years ago, they would go to the pub, now they come in and shoot some people and go home again."

People can play games for a rate of £10 for three hours and the centre takes bookings - it has hosted the odd stag do. McMahon admits that games in general appeal more to men, but a couple of Ministry's regulars are women.

Sam, from Glasgow, who is in her thirties, is one female devotee. Her guild, Tribe, has 40 or 50 members. Of the core group of regulars, five or six are women, including the guildmaster. The members are mostly in their late 20s and 30s with full-time jobs and get together once a year in Sweden. "They arrive from all over Europe," she says. "Quite a lot of the guys who play work in IT, so it is a bit of a geekfest, but it's fun." The group discuss WoW, of course, but their friendships now go beyond that.

She can see how someone would get obsessed, but for her, it's just another hobby, limited to a couple of sessions a week. "I work full-time, I go to the gym four nights a week and I don't watch soaps or anything like that. I haven't got the new expansion set because I've been too busy this week. Thursday I was at the gym and the cinema; Friday I'm out at a do; if it doesn't arrive on Saturday morning, I won't be stalking the postman."

The world of warcraft