I remember the first time I forcibly medicated a person against his will. It was 13 years ago, not long after I’d qualified as a mental health nurse, and I had started my career working on a psychiatric ward providing assessment and treatment for adults in acute phases of serious mental illness.

There was a patient (or service user or client or son or brother or friend, depending on who you ask) whom I’ll call Amit. Amit had been refusing any medication for nearly three weeks and with good reason. The medicine we were offering him contained a poison. It had been prescribed by a doctor who wished to harm him. In fact, this doctor – a consultant psychiatrist – had been struck off the medical register for his abuse of Amit during previous admissions and so was now working illegally on the ward. Many of the nursing staff knew this, and were in on it.

During morning medication round, Amit stood in the doorway of the ward clinic, watching me closely. He watched the movement of my hands over the drugs trolley as I secretly replaced his regular tablets with harmful ones.

He was wearing the same clothes that he’d slept in and a pair of old trainers, one with a huge split down the side. Amit knew the water supply to his room was deliberately contaminated and so hadn’t washed since he was admitted. I would try to talk to him about that again later – to find the right words – but for now, at least, the medication was the priority.

I double-checked the dose on his chart, put two tablets into a clear plastic pot and held it out for him to take. He stared at it. We both did. I tried some words of reassurance. “I know you’re finding it hard to trust us at the moment, Amit. I do understand that. We think that’s all a part of you being unwell again.” He knew I was lying. “I’ll take them in my room,” he said. I knew he was lying. “You know it doesn’t work that way. I’m sorry, but I need to see you take them.”

He cautiously reached out and took the pot from me. He prodded at the tablets inside. His fingers were stained dark yellow from tobacco. “Nah. You’re all right,” he said at last, placing the pot on top of the drugs trolley and backing out of the clinic, watching me the whole time. As he disappeared down the long corridor towards his bedroom, I wrote an “R” for “refused” on his medication chart. Of course he refused. Why wouldn’t he refuse? If I were in his position, I know I would.

But I don’t know if I would refuse with the same dignity he showed when later that afternoon the C&R team entered his bedroom.

C&R. Control and restraint. The legal (if controversial) techniques that mental health nurses are trained in to render a person unable to fight back. In subsequent years, this training would be rebranded as “prevention and management of violence and aggression”, which is reasonable if a person is smashing up the ward or threatening to hurt someone; but at times like this, for my money, the first description felt more honest.

It had been decided in a team meeting that this was the last day Amit could refuse oral medication before we would use an injectable form. In the parlance of psychiatry: his mental state was deteriorating daily; he was well known to mental health services; and this was a typical presentation and pattern of his illness. If we could get him back on a stable dose of medication he’d likely respond well.

Amit was sitting on his bed, smoking and tuning through the static on a portable radio. He was talking to somebody that none of us could see. He looked up. There were five of us.

“Do I have to beg you?” he asked.

There is no uncontroversial language when talking about mental illness – and that includes the phrase 'mental illness'

A colleague of mine explained his options, such as they were. But that’s the bit that stayed with me. Do I have to beg you? It’s why I struggled to keep my hands from shaking as he was eventually held down on his bed and I administered the injection. He didn’t put up a fight. We weren’t preventing and managing violence and aggression. From Amit’s perspective, I don’t doubt we were perpetrating it. In that moment, however good my intentions, I was knowingly participating in his suffering.

It was around this time that I began to try to write a novel. (There’s a nice Peter Cook quote that pretty much sums up my experience of this: “I met a man at a party. He said, ‘I’m writing a novel.’ I said, ‘Oh really? Neither am I.’”) I was living in a small flat in inner-city Bristol, and between shifts on the ward I would sit for long hours at my desk, imagining the life of a young man who was suffering from the symptoms of a strange and commonly misunderstood illness (or disease or condition or trauma or phenomenon or curse or gift, depending on who you ask), and also the lives of this man’s family and friends. For me, understanding what this character was going through was an imaginative exercise. Or put another way: an act of empathy.

This is something that writing a novel and mental healthcare have in common. To do each of them well requires bucketloads of empathy; of striving to understand the feelings of other people. Of course, as an author of fiction I was also responsible for creating the very problems that I then had to empathise with. Though, thinking about Amit, I probably did that as a nurse quite often too.

I decided not to diagnose the character in my novel, but if I had, I’d have probably landed on “schizophrenia”. What a word, huh? It’s derived from the Greek skhizein, “to split”, and phren, “mind”. Small wonder then that the perception of a divided person with two or more distinct personalities has endured so immutably in the public imagination. It’s utter nonsense though.

‘Psychosis may be a major feature of so-called schizophrenia, but it’s by no means the whole picture’ ... Nathan Filer. Photograph: Sarah Lee/The Guardian

Let’s be clear about this from the start: schizophrenia does not mean split personality. Neither does it mean multiple personality. But declaring what it isn’t is a good deal easier than asserting what it is. There’s credible and often heated debate across the fields of psychiatry, psychology, genetics, neuroscience and various mental health charities and campaign groups over everything from causes and risk factors to categorisation and treatments, and indeed as to whether the diagnosis has outlasted its usefulness (if it ever was useful) and should be rebuilt from scratch or abandoned entirely.

If we tentatively take a seat in this debate, the first thing that will become clear is that there is no uncontroversial language when talking about mental illness – and that includes the phrase “mental illness”. On the whole, the controversy around a term tends to relate to how medical it feels. Take the collective noun for people accessing treatment. If you’re using mental health services and subscribe to the view that your distressing thoughts and feelings are an illness, presumably located within your brain, and essentially the same as any physical illness, then you might prefer to think of yourself as a “patient”. After all, if you’re the same as those patients receiving care for broken bones and pneumonia and cancer and diabetes and chest infections, then why should you be called something different?

However, if you’re of the view – shared by many people, including health professionals – that even the most alarming of your thoughts, and the most extreme changes in your moods, and your most uncharacteristic behaviours are not symptomatic of illness so much as a natural response to undischarged trauma or painful life events, or even poverty, then to see this wrapped up in a medical language that begins with you becoming a “patient” might feel seriously problematic. It was during my own nursing training that the currently favoured term of “service user” gained traction, as it was deemed more neutral. But what about people like Amit? People who are detained in hospitals and medicated against their will? Does the collective noun of service user really cut it for them? Can we in all conscience say they are using mental health services? Probably not.

Today there’s a growing minority of people who eschew both terms and collectively self-identify as “survivors”, while the council of the Royal College of Psychiatrists recently recommitted to “patient”. And if all this sounds complicated and fraught with politics it’s because it absolutely is.

The language of diagnosis, for better or worse, has the power to profoundly alter people’s lives

As with any impassioned debate, there are almost certainly elements of prejudice on all sides. That said, it would be a grave mistake to dismiss any of this as unimportant. Yes, it’s a dispute about language. But in the world of mental healthcare, language is everything. A simple truth, not widely appreciated, is that the overwhelming majority of mental health diagnoses aren’t arrived at by looking at blood tests or brain scans or anything of the sort. They can’t be. These tests do not exist. Rather, it is the words people say – or do not say – as interpreted by professionals, that as much as anything else will determine a diagnosis.

And the language of diagnosis, for better or worse, has the power to profoundly alter people’s lives. If something as seemingly innocuous as the word “patient” is the subject of such controversy, we can now begin to imagine the dark storms of debate swirling around the truly immense subject of “schizophrenia”.

This mysterious diagnosis with its misleading name has traditionally been seen as the heartland of psychiatry; the condition that defines the discipline. This heartland is also the battleground on which the fiercest ideological disputes about madness and its meanings are fought. Believe me when I tell you these disputes are fierce. Curiously, a great deal of this acrimony exists between two professional guilds that work closely alongside each other, and that many people assume are one and the same thing. I’m talking here about the distinct but related disciplines of psychiatry and psychology.

But of all the “psych-” words, the one most loaded with popular misconceptions is psychosis – generally considered to be a defining feature of so-called schizophrenia. Though not an especially precise term, at its broadest and most simplistic psychosis describes the phenomenon of a person losing contact with reality – or, at any rate, losing contact with what most other people perceive as reality.

It’s not considered to be an illness or disease in and of itself, though it can certainly be symptomatic of disease. It’s a typical feature in most forms of dementia, for instance. Many of us will experience psychosis at some point in our lives; we may even actively pursue it. It’s the desired effect of numerous recreational drugs. If you try LSD and it doesn’t radically distort your experience of reality, then I suggest you find a new dealer.

Importantly, what we call psychosis can also be a response to extreme stress or trauma. For many people it might best be understood as a kind of psychological adaptation, a coping strategy gone awry or a form of storytelling carried out within the mind as a response to unbearable life events. Whatever its cause, psychosis is commonly experienced through hallucinations and delusions. Amit’s belief that we were conspiring against him could be described as a delusion. It might also be described as an understandable response to what was happening to him.

Most people who are diagnosed as having schizophrenia experience this kind of detachment from reality. Often – though not always – this is deeply distressing and can lead to strange behaviours, as the person tries to navigate and survive in their altered, hostile world.

Psychosis may be a major feature of so-called schizophrenia, but it’s by no means the whole picture. Other symptoms can include: a disintegration in the process of thinking; disorganised speech; disorganised behaviour; flattened or incongruous emotional responses; impaired attention and significant social withdrawal. In a popular TED talk, Professor Elyn Saks, an expert in mental health who herself lives with a schizophrenia diagnosis, asserts: “The schizophrenic mind is not split but shattered.” It’s also a surprisingly common phenomenon. A statistic bandied around for years is that worldwide it affects around one in every hundred people.

It’s not always possible to find the right words but we can still be part of the conversation. We can walk with people for a bit, sit with them, hear them. Photograph: Stephen Burke/The Guardian

A mere nine years after I’d first sat in front of my computer to stare hopelessly at a blank page, my novel, The Shock of the Fall, was – by some miracle – finished. In that time, I’d left frontline nursing to work in mental health research at the University of Bristol. I’d also had a baby daughter, got married, and was wondering whether I should maybe try to write another book one day. Then the emails arrived.

They were from people I’d never met but who had read my fictional account of a young man living with “schizophrenia” and had taken the time to share their own stories. Many were upsetting, others hopeful. Rarely did they have the kind of neatly conceived beginning, middle and end that as a novelist I had the luxury to craft. A truth about the strange phenomenon we call mental illness is that it’s messy and chaotic; it can be extremely difficult to make sense of, but that doesn’t mean we shouldn’t try. There’s a fragility to the mental health of everyone. It serves us all to be part of the conversation.

I realised I needed to think more about such concepts as stigma (and why anti-stigma campaigns may be missing the point); psychiatric diagnosis (and why the science behind this is deeply flawed); the causes of “mental illness” (and how sometimes what needs “fixing” mightn’t reside within the individual at all); delusions and hallucinations (and how these are a part of all of our lives, all of the time); and psychiatric medication (including cracks in the evidence behind current prescribing practices).

On my first day of work in a psychiatric hospital, I spent most of my time sitting in a dreary smoking room drinking tea with the “service users”. Someone took a long drag of their cigarette and told me that before they came on to the ward they hadn’t known such places really existed. I didn’t know what to say, which by chance meant I probably did the best thing. I listened. It’s not always possible to find the right words but we can walk with people for a bit, sit with them, hear them.

• This is an edited extract from The Heartland: Finding and Losing Schizophrenia by Nathan Filer, published by Faber (£14.99) on 6 June. To order a copy go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £15, online orders only. Phone orders min p&p of £1.99.