When does passionate love spill over and become unhealthy obsession? Claire Seeber investigates
The week after I split up with William, I called round to see my best friend. She came to the door flushed and quite obviously hiding something from me. I didn't know it was William."

Jodie, down-to earth and delicate, learnt the meaning of obsession when she finished with her boyfriend. After years of abusive behaviour, leaving him was a struggle - she loved him, but she couldn't live with his neglect any longer. William, however, was determined to get her back. Unfortunately his idea of proving his love to Jodie was fuelled by an obsession that no one else should have her. He scared off her potential lovers with physical threats, attacking one guy in a club, and then systematically began to seduce all her friends. He wrote her poetry - when he'd never written her so much as a Post-it note before - and hounded her socially.

"He got into my house and read my diary - which was hidden. He'd ring me and quote whole chunks back to me," Jodie says. "Then suddenly every time I went out I was met with gossip involving William and his sexual exploits - with my friends. If he couldn't have me, he wanted to rub my face in it. I didn't want to be with him, but I didn't want to keep turning up to places to see him with girls I knew. He was trying to make me jealous and stay in my life any way he could.

"Finally I met someone I really liked and didn't see Will for a while. He turned up at my house really pissed when Jason, my new boyfriend, was away and started screaming that Jason had been sleeping with loads of girls behind my back. I didn't believe him, but it planted seeds of doubt in my mind. It was all so warped."

He had become totally obsessive, refusing to accept it was over simply because he didn't want it to be, and until Jodie moved away, it continued. "The irony was," says Jodie, "if he'd only been half as interested in me when we'd been together, perhaps it would have lasted."

"Love is the sweetest addiction", or so Erica Jong once declared. At least it is until you can't have it any more, and then it is perhaps the most bitter of pills. Recent studies suggest that love is triggered by chemical hormones in our bodies and that it is, like all things nice, addictive. But at what point does acceptable, everyday love spiral out of control and into obsession?

Hollywood has long been the playground of the great lover, but even money and status can't quell a fixation. Singer Sheena Easton recently split from her third husband, director Tim Delarm, a year after they'd eloped. Thrice married Sheena filed for divorce, citing irreconcilable differences and denying Delarm "spousal support". He, by all accounts, refused to accept it was over and, last month, she took out an injunction to keep him away from her. In a recent letter to fans, Easton reassures them that "a lot of therapy" is helping her get through "paying for my mistakes".

So what pushes someone otherwise quite sane into the depths of desperate obsession? Well, fiction, like film, has always taught us that love is blind. The Ancient Greeks believed love was a form of insanity, but from Wuthering Heights to Mills and Boon, we are indoctrinated by the belief that to love unconditionally is aspirational. Cinderella's prince was so infatuated that he was prepared to traverse the kingdom smelling feet until he found the object of his desire, whilst Snow White's kissed a corpse. Heathcliff was undoubtedly obsessed by Cathy, so that even his life after her death was dictated by his love. And Thackeray's Vanity Fair, recently adapted by the BBC, centred around the fair Amelia Osborne, so fixated by the memory of her late husband that she ignored all indications of his unfaithful heart and refused to love another until the spell was broken more than 10 years later. It is not surprising that some of us step over the thin line and into obsession.

Despite our universal celebration of love, we are expected automatically to know our boundaries. The man serenading his love under her window in a sleeping street is a romantic - not so the man who stalks his ex wherever she may go. The first stages of love are meant to be wild, inexplicable and exhilarating. But then it's meant to settle into something more calming - or fizzle out. It is not meant to be totally out of control.

So what drives the obsession? An anxious state, a desire to own, complete delusion, a refusal to accept that something is over. A denial of rejection or change. The obsessed cannot think of anything but their object of desire. It becomes their life's focus. And depending on that person's nature the obsession can be anything from mild to menacing.

Dr Niall Campbell, consultant psychiatrist at Tolworth Hospital, Kingston- upon-Thames, suggests that it is not literature or even society to blame for obsessive love, but the individual. "We believe it's the norm to be in a relationship, but some people can't do it," he says. "The person who becomes obsessed is usually dysfunctional or empty in some way, whether it's an insecurity caused by unstable role models during childhood or previous bad relationships. Displaying inappropriate jealousy or obsessive behaviour is not psychotic, but more likely to be triggered by a personality disorder - some sort of problem coping with life and/or relationships ."

The tragedy of obsessive love is that it's often the very obsession which drives the beloved away. Carol met Oliver and everything was going well until Oliver started to become obsessive about her. "I couldn't understand why," Carol says "but he suddenly became really paranoid. We were commuting to see each other, and Oliver would write really long vivid letters, full of drawings of me. When he came to stay, he'd expect to see the letters pinned above my bed. Then during one visit we went to the pub with Pete and Joe, my flat-mates. As we walked home Oliver took a key to Pete's new car - just because I'd mentioned Pete's name.

"The next day I went shopping. I returned to find Oliver had burned every single photo of me with my ex, in the middle of my floor, plus a shirt that I'd been wearing in one of the photos. After that I started to feel scared of him and when I went away to college, I called it a day."

Dr Campbell suggests that the obsessive type is "prone to other negative emotions, such as depression or feelings of inadequacy, and likely to be very black and white about relationships. They will have unrealistic ideals of any affair whilst simultaneously looking for something to go wrong." Carol was fortunate that Oliver accepted things were through - often the end is only the beginning of the trouble.

The last woman to be hanged, Ruth Ellis, famously depicted by Miranda Richardson in Dance With A Stranger, was so haunted by her selfish lover that the only way she could be sure to cure the problem was to shoot him. We all know the story of Othello, whose obsession became pathological, deluding himself to such an extent that the only relief was to kill the woman he loved. Modern-day cases of such jealousy include men who have installed secret mirrors to spy on their wives and one City broker who carried a machete in his briefcase to use against his wife's imaginary lovers.

But the most serious form of obsessive love comes in the form of De Clerambault Syndrome, or erotomania. This is the belief that someone, usually unattainable - ie your doctor, lawyer or even Rod Stewart - loves you passionately - although you may not feel you love them back. It's very often a female illness and is generally accompanied by other problems such as depression, schizophrenia or alcohol-related disorders. Sufferers will pester the object of their desire - and it's the form of obsessive love most likely to turn violent when the victim feels rejection.

Dr Campbell maintains "there is no real connecting cause between cases of obsessive love". It is down to the individual. So if you're at the end of your tether when your ex has just turned up outside your window again, the best thing to do is "make no contact. No communication whatsoever - the obsessed will only thrive on anything you feed them, and will misinterpret your words or actions to suit their delusion. If the worst comes to the worst, take out an injunction."

Not every tale of obsessive love ends in high drama. Like Proust's hero Swann, who obsessed about his courtesan until he married her, obsession can sometimes be simple delusion, fuelled by nothing much better to think about. So if you've ever caught yourself dropping your loved one's name needlessly into conversation, just for the thrill of hearing the word, do not fear. Plenty of us have rung answerphones just to listen to those special dulcet tones, and some of us have written unposted letters to the one that jilted us. It's fine to wallow in memories of when it was still good as long as you can start afresh when your next lover comes along. The day, however, that you wake up and find pleasure in the pain of obsession, is the day to seek help.