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THE MUST-READ BESTSELLER AND BOOK OF THE YEAR IN THE GUARDIAN, SUNDAY TIMES, FINANCIAL TIMES, ECONOMIST, NEW SCIENTIST AND MORE

‘Revelatory. Slices through the contradictions.’ – CHRIS VAN TULLEKEN
‘So well-written… critically important.’ – RANGAN CHATTERJEE
‘A doctor’s brilliant study of the dangers of overdiagnosis’ – GUARDIAN

From autism to allergies, ADHD to long Covid, more people are being labelled with medical conditions than ever before. But can a diagnosis do us more harm than good?

The boundaries between sickness and health are being redrawn.

Mental health categories are shifting and expanding all the time, radically altering what we consider to be ‘normal’. Genetic tests can now detect pathologies decades before people experience symptoms, and sometimes before they’re even born. And increased health screening draws more and more people into believing they are unwell.

An accurate diagnosis can bring greater understanding and of course improved treatment. But many diagnoses aren’t as definitive as we think. And in some cases they risk turning healthy people into patients.

Drawing on the stories of real people, as well as decades of clinical practice and the latest medical research, Dr Suzanne O’Sullivan overturns long held assumptions and transforms how we think about illness and health.

‘Compassionate and bracingly independent thinking’ – THE TIMES
‘Exceptional. Chapter by brilliant chapter, it raises fundamental questions we should all be asking.’ – NEW STATESMAN

*As heard on Radio 4 Book of the Week, Today, Good Morning Britain, Sky News and more.*

Publisher Review

The Age of Diagnosis covers so many topics that have been troubling me but which I hadn’t been able to resolve myself. It slices through the confusion and the contradictions that have tied me in knots – both as a parent and as a clinician – with grace, elegance and compassion. It is scholarly and human, but an absolutely absorbing read from start to finish. There are very few people who could write this so straightforwardly and yet with endless compassion. I really cannot say good enough things about it. — Chris van Tulleken, author of ULTRA-PROCESSED PEOPLE O’Sullivan is brave to take this subject on, and she hits the target… she is an excellent, fluid writer, and an eloquent speaker… In a world where medical misinformation and disinformation flourish, and people die as a result, it takes courage to counter them without pandering to stereotypes. But that is what The Age of Diagnosis does so well… Its overall message is clear: diagnosis is a tool to be wielded with the utmost caution, and tolerance for difference and for imperfection can go a long way in keeping us healthy. — Adam Rutherford * The Guardian * ‘Exceptional… Chapter by brilliant chapter, [The Age of Diagnosis] raises fundamental questions we should all be asking when thinking about illness, be it cancer or genetic disorders, never shying away from difficult truths.’ — Hannah Barnes * New Statesman * ‘The neurologist takes eloquent aim at a medical culture that, although well intentioned, is too quick to assign clinical labels to aspects of the human condition… A brave and compassionate book.’ * Financial Times, best summer books of 2025 * How does she carry off that high wire act, being so technical and expert and grounded in data, while telling stories in a way that makes them utterly addictive? I love the way she listens to people talking. It’s a really important, intelligent, compassionate and brave book, and it speaks very eloquently to things happening in our ideas about ourselves. — Tessa Hadley A humane and thoughtful observer of the oddities of the human mind… full of intriguing case studies and wise observations. Her new book ranges widely, taking in the drawbacks of mass screening for illnesses as well as the perils of overextending mental health categories… We make people sicker by by the simple act of diagnosing them with a medical problem. A fascinating book. — Robbie Millen * The Times * A brave and deeply compassionate book with a very important message. — Henry Marsh The neurologist Suzanne O’Sullivan has written insightful books about psychosomatic conditions. Here she turns her attention to the perils of over-diagnosis – including autism, allergies, ADHD and long Covid. Be prepared for compassionate and bracingly independent thinking. * The Times books to look out for in 2025 * In my view the best science writer around – a true descendant of Oliver Sacks. — Sathnam Sanghera Two elegant offerings from neurologists stand out for using patient stories to tell us something about ourselves. Suzanne O’Sullivan courageously questions medicine’s well-intentioned enthusiasm for attaching labels – think ADHD, anxiety – to aspects of the human condition. This is sensitive political territory, given the public conversation about the 2.8m people who are economically inactive due to long-term illness, but it deserves a hearing. * Guardian, Best Books of 2025 * A book of great wisdom as well as compassion – the result of decades working along the frontiers of brain, mind and body. Modern medicine is powerful; with care and with stories from her clinic, Dr O’Sullivan shows just how harmful it can be too. — Gavin Francis, author of ADVENTURES IN HUMAN BEING I loved this beautifully written and provocative book. The Age of Diagnosis asks brave and important questions, taking on the medical establishment as well as contemporary societal norms. O’Sullivan’s primary motivation in her writing, her philosophy and her work as a doctor is extreme empathy and advocacy for her patients. — Christie Watson, author of THE LANGUAGE OF KINDNESS O’Sullivan explodes conventional wisdom about medical diagnoses. She knows that having a diagnosis like austism, or ADHD, or depression, or Lyme disease, can give her patients relief at having an explanation. But she also invites us to wonder where these diagnoses came from. Are we overdiagnosing? Is this really the best strategy for helping people get better? With clarity of prose and reasoning, The Age of Diagnosis should make all of us think about whether we are more or less healthy when we receive a diagnostic label. — Elizabeth F. Loftus, Distinguished Professor, University of California, Irvine ‘O’Sullivan impressively marshals evidence of how drivers of ‘overmedicalisation’ have arisen, the minimal benefits conferred and the harms that can result. Most striking is her examination of the explosion in diagnoses of mental health conditions, such as autism and ADHD… O’Sullivan seeks out the ‘reality’ behind appearances, though her scepticism is tempered with sympathy and understanding… So entrenched has overmedicalisation become that there now seems no prospect of its being reversed. O’Sullivan’s masterful diagnosis of how this has come about is immensely persuasive.’ — James Le Fanu * Literary Review *