The first time Jane cried openly in public was on the anniversary of her car crash. Instead of going to the pub with the two of her friends who survived, she found herself drawn into a church service and, "Oh my God", she says, "the local children's choir began to sing, and I had to be helped out of my pew".

For Jane, it had been a year in which everything familiar became strange. The woman who had strapped herself into her Saab saloon one day in April 2005 had been a confident magazine editor; the kind of boss who supported others when they cried. The bloodied figure who staggered out an hour later, to search amid the wreckage for survivors, would come to view her life as a charade.

For a while, she held it together. Then, one night in September 2005, Jane had an experience that she says was like being taken over by another being. "Something came from deep inside that made me wake up screaming and wiped me out for two months. I'd always thought I was good at coping, that I was in control - that I was 'me'. But I'm not. There's something in there that I don't understand."

Jane found a psychiatrist, privately, who gave her cognitive therapy. This seemed to help, particularly in the way it provided a simple explanation: that her psychological disorder was part of a natural, albeit extreme, mental mechanism that was still processing her traumatic memories. The psychiatrist also prescribed an antidepressant. Jane asked if this would muddle the "memory processing", but the psychiatrist assured her that the drug - 20mg of citalopram daily - would scarcely interfere; it might simply ease things along. He did warn that she could experience some side-effects; but Jane noticed none. In fact, she didn't notice any effects at all.

Perhaps her post-traumatic symptoms would have been even worse if it hadn't been for the citalopram. She can't be certain. What she knows is that, for nine months, she wrestled with nightmares of being pursued by malevolent figures, and "daymares" of struggling with work, which had come to feel nauseatingly pointless. Magazines and papers seemed to be filled with nothing but inane junk. All that helped were chats with her psychiatrist and the companionship she found in a survivors' group.

Then came the anniversary church service. "It was as if the old Jane had come back to life," she says. She still felt the same about the pointlessness of her work, and a fear of being single at the age of 38, but the mood had changed. She could manage. The relation of emotion to mood is like that of weather to climate. Jane's London was still drizzling, but now she was back in a familiar, temperate, British climate.

So, without consulting her GP or psychiatrist, she decided to wind down the citalopram. In September last year, she reduced the dosage from 20mg to 10mg. For two weeks, she felt fine. Then she plunged into the first real depression she had known. It wasn't like the dark terrors she had experienced in the year after the crash; this was a deep, paralysing despair. She had been warned about side-effects while taking her antidepressant, but not possible withdrawal symptoms.

Then Jane noticed something odd. The date she plunged into the depression, apparently due to reducing her dose, had been September 30 - what would have been her best friend's 40th birthday, had she survived the car accident. Not that she'd been aware of it on the day. What was going on? Was she withdrawing from a drug, or reacting to some peculiar psychological trigger?

Yet, after six weeks on 10mg, the climate changed once more. Her depression lifted and Jane "came back" again. She dropped the dose to 5mg. The same thing happened: two good weeks, six weeks of abysmal depression, then a levelling out. Citalopram is generally understood to be one of the cleaner types of SSRI antidepressants, but Jane learned that between 10% and 30% of people may experience withdrawal from SSRIs. Perhaps she was one of the minority to suffer in this way. She found some comfort in sensing that a pattern was emerging. If she could take herself down from 5mg to zero, perhaps she would be free - from withdrawal, from depression, from everything.

In January, she stopped taking citalopram. Instead of a psychological depression, this time she experienced new symptoms: shivering, dizziness, headaches, vomiting, exhaustion. There was something oddly reassuring about this. If the effects were now physical, she could get over them. She resigned from her job and booked a six-week trip to south America. She left this month, feeling no particular optimism, only a certainty that, whatever the weather, the climate would never be the same again.

· Names and details have been changed.