Emma was the cool kid in class who made Natalie Meddings feel special. They grew up together: shopping, partying and sharing a house. Who could have known their relationship would come to such a shocking end?

The year is 1981, and I am 14. I have bought my first eyeliner (blue, Boots No.17) and better still, I have a new friend. A good new friend, I think. She is called Emma. We have been in the same year since we were 11, but it is only now, with our newly allotted places in the O-level biology group, that we find ourselves desk to desk, side by side, and a new and warm connection forms. We are neither of us especially outgoing, and since I know that 1) Emma is pretty and therefore popular, and 2) she reads Jackie with a gang of trendier sorts at breaktime, I am cautious, conscious of my lack of "credentials". The only boys I know are my brothers and the excitement that is shopping in Miss Selfridge on a Saturday is as yet unknown to me. But it doesn't seem to matter.

"Those are nice," she says, pointing with plain admiration at my turquoise "holiday" earrings, and when I am equally ' impressed by her own hippie market find – a silver ring on her right middle finger – we are off, talking excitedly of Rhodes and Ibiza, discos and suntans, and Wham!.

Before long, biology is a class I look forward to. Because of Emma. With her, I can talk in a way I never have before. Thanks to her lovely way of listening, her eyes that seem to take me seriously, I am able to cross into new, grown-up ground. We whisper and giggle and have a terribly good time as we distractedly dissect frogs and haphazardly annotate our drawings of the human heart, and the truth is I just feel good around her – more like myself.

"I'm going to be a nurse," she tells me one day, all sensible and sure and I think, "Yes you are, of course you are," because she is kind and careful with people in equal measure. Making people better is something Emma is made for.

By the time that she was a nurse, our friendship had become the solid sort – like a seaside groyne sunk deep in the sand. We each had other good friends, but Emma was always a source of something else for me, a mainstay.

We'd marked all the usual teenage milestones together and then, as good fortune would have it, had ended up in the same city to do our degrees – two painfully squeaky Surrey girls in wind-blown Leeds. Oh what comfort, leaping into life proper alongside someone you like and know. And what pleasure too, sharing a "first" house, scavenging jumble sales for the 1950s frocks we liked so much, dancing until our feet were sore in gloriously innocent pre-E clubs.

We were happy – really happy, I thought. But by now, Emma was growing used to the pull of something else too. She had always enjoyed a good "cry", as she would say, but now I would often look up to find her face wet, tears inexplicably spilling down. Lows which had been brewing by the end of school would now gust with more force – and though to me, her deep-down self seemed as sweet and artless and easy as it had always been, Emma was diagnosed with depression.

Like so many sufferers, she seemed far from the "sort". What reason could my bright and beautiful friend have for these sudden episodes of suffering? According to Emma, there was no reason, a fact she would always use against herself, mixing self-blame in with the anxiety, inadequacy and indecision that would dog her on and off for years to come.

They would come, those sleepy, edgy feelings, then they would go, come then go again, blowing in like a patch of bad weather. To chart their path as they wound their way through and around Emma's life is pointless. Suffice to say, the depression sat inside her undecoded, at times taking and claiming her, turning her into someone troubled and hesitant. And then giving her back to us, rosy-cheeked and ready for life.

By her late-twenties she was an experienced nurse, involved in a relationship and living in Brighton. Anti-depressants were an intermittent prop, but she was always Em, and sitting with her – her cat on my lap, tea in our hands, talking until our faces ached and the light outside changed – was still one of the nicest things I knew.

She'd been so steady for so long now, her depression seemed something distant to us all. But then, while travelling – a trip through Asia and Australia she'd wanted to do for years – Emma nosedived. She came straight home, and to see her was awful: the hesitance after all her traveller's hopes, her face full of forced smiles, her attempts to conceal the unspeakable freefall she was feeling. She would wake early and worry – about what, she did not know – and rattle with a kind of restlessness that no amount of reassurance could reach.

She began attending day hospital, was stabilised with a fresh and more powerful dose of anti-depressants and, after a few dark months, reported that she was coping, she felt well. For my part, she'd never seemed less so. Her eyes were glassy, her behaviour accelerated and her demeanour a cocktail of mannered tics.

They were little things, but they were unfamiliar things: a cocking of her head as she talked, a tucking of loose hair behind ears, uncharacteristic swearing and chain-smoking, as if she was a person in a play, performing "nonchalance". Her new and shiny self was unsettling to be around; a soma-taking character from Aldous Huxley's Brave New World. My friend just didn't feel real. Her dose was adjusted, her mood levelled out and, after some time, the general consensus was that Emma had returned. But to me she seemed rearranged. I could feel some kind of nothingness inside her.

Evidence of a genuine connection with a person is how you feel when you part – your head thrumming with talk talked, your heart full of feelings felt. Now, when Emma exited, there was an emptiness. She left no mark.

Peace, it seems, has its price, and she – we – were all prepared to pay it. Sensitivity, concentration, involvement – such human fine-tuning has to be sacrificed if anti-depressants are to do their work and recover a person's sanity.

And recover it they did. Emma's now largely unshareable self was nevertheless "normal", capable of ordinary living. She excelled in her work as a nurse on an Aids unit and established a strong and loving relationship with a new man, John. The couple bought a home and got ready for a future in it. For five years she felt that way: good – even great. And then, when she was 35, she decided she wanted to have a baby.

It meant coming off her medication. The effects of such drugs on pregnancy simply weren't certain enough and with as much optimism as she could muster, Emma started to lower her dose. "I'll be fine, I've been so well for so long now," she reassured herself and those around her.

But then, just a few weeks in, Christmas 2002, Emma reported feelings of her former anxiety. She told me this in a small voice, with a dry mouth, like someone who feels frightened. "I can't bear to go back to that," she said, "I just can't." And with an abrupt about-turn, she resumed her usual dose.

I got the call just 10 weeks later. It was from a hospital in Sussex. Could I confirm that I was a friend? Could I come straight away? John had been unable to find contact details for Emma's family, and for the time being, I was needed.

All questions were deflected. "We can't discuss it on the phone," they said and I knew at once that Emma had tried to take her life. She had done it once before, a dozy half-hearted attempt at an overdose. "I just wanted to go to sleep," she'd said after. And I guessed the same had happened again.

In the car on the way down from London, I prepared a speech in my head – nothing admonishing, words of love and acceptance. I even imagined us being able to talk a little, finding the thread we'd always shared and following it.

When I arrived, Emma's sister Rebecca was standing alone on a small circle of lawn in front of the hospital. I raced up to find her face colourless. "Oh Natalie," she said, closing her eyes, not wanting to make it realer with words.

That morning, at around 9am, Emma had driven six or seven miles from her home in Brighton to a small Sussex town nestling at the foot of the downs – a quaint little place with a tearoom and a post office and a proper green. Emma drove past all this, past the people going about their ordinary Monday business and parked up in a side-road. She then poured a can of petrol over her head and body and set light to herself.

On fire, she started to run along the road. In distress? Having suddenly come round to what she'd done? We don't know. What we do know is that a man stopped and helped – doused her with a bottle of water and called an ambulance. We know, too, that she spoke to him, some sort of sorry. Words of worry about her mum. Coherence in any case, and it is these moments that torment me the most. The knowledge that she was sitting on a pavement, aware and with it. For months to come I would find myself longing to slip through space and time and sit next to her, speak with her, ask her why. I would have told her how much I loved her.

By now Alan and Heather, Emma's parents, had been contacted and were on their way down. John rose to meet me as I walked in and we wordlessly grabbed each other. I'd always thought of shock as a sudden thing, but this kind was seeping into each of us, making movement hard, limbs heavy and stiff.

"Do you want to see her?" Rebecca asked. "She's unconscious but we could go in together." And so we did.

I'm not going to spare you. Not while there is a small chance that a drug-company executive might be reading this. Maybe, as has been done to others with a history of depressive illness, he or she will dismiss my friend. "It would have happened anyway..." they'll think. Maybe that's what you think. Or maybe you will find the courage to reconsider. To believe those who knew and loved her when they say that Emma was not psychotic, not a crazy person. Emma would never, ever, even at her most despairing, have done a thing so desperate.

She had withdrawn from the drug, she had gone back on the drug and I believe the sum result in her brain was biochemical catastrophe. An explosion in her mental circuitry. It is the only way to explain such an uncharacteristic act of violence. Half her hair was gone, her face was bloated and misshapen, and she was covered in clingfilm. She had a plastic tube coming out of her mouth and her eyes were tightly swollen slits. We couldn't touch her so I reached for Rebecca – and that was awful. Because her body was so familiar, it felt like Emma in my arms.

We spent the afternoon in a detached daze, Emma's family, John, me. Half-thinking there was a chance, especially when they said they were transferring her to a specialist burns unit. But on arrival, they merely made her comfortable. There was nothing they could do. There were no tubes now, no life-support. It was just waiting. I thought it would be hours and started phoning friends, asking them to hold her in their mind, to transmit love and comfort. And then a nurse gave me a tap. "You need to go now," she urged.

The room was dim and warm. Emma's scalded face and body had ballooned and you couldn't recognise her. A minute passed, then another – then the nurse nodded. She'd gone.

The sudden fact of Emma's absence was dizzying, unbearable enough to knock the breath from me. I remember doubling over, having to steady myself on the end of her bed.

Outside, it was dark – this day had passed and altered everything, and under the bright hospital lights we all of us just stood there, staring at each other, understanding neither what had happened nor what to do next.

Her poor parents, her lovely sister, her sweet and gentle boyfriend, eventually left. And then I followed them out, into the strangest of things. An unthinkable place. A world without my friend Emma in it.

www.mind.org.uk, 0845 766 0163