He had beaten more than 19,000 applicants for a place at medical school, yet Khurram Sadiq was now bunking off his hospital shifts. The 19-year-old felt inexplicably anxious around strangers on the wards and was hiding from his own patients.

During lectures he couldn’t focus on what he was being taught. He deemed himself “a goof, a dunce” in contrast to his peers. Sadiq couldn’t motivate himself to revise for his exams and instead found himself panic reading textbooks in the final days. He passed his undergraduate pre-medical exams by the skin of his teeth.

That was 30 years ago. In the decades since, Dr Sadiq has qualified as a consultant psychiatrist, been diagnosed with both autism and attention deficit hyperactivity disorder (ADHD), specialised in autism and ADHD psychiatry and met hundreds of patients with struggles similar to his. He is now trying to spread what was once an unbelievable message: that both autism and ADHD can coexist in the same person simultaneously.

Just over a decade ago, the two conditions were considered to be mutually exclusive, with the Diagnostic and Statistical Manual of Mental Disorders, often referred to as “psychiatry’s bible”, stating that the diagnosis of one precluded the existence of the other. This wasn’t revised until 2013. “It led to a fork in the road,” says Dr Jessica Eccles, spokesperson for the Royal College of Psychiatrists. “Not only for clinical practice, but also for research and public understanding of these conditions.”

Now some specialists believe that the coexistence of both conditions is not just possible, but frequent. One study by researchers at Duke University found that up to half of people diagnosed as autistic also exhibit ADHD symptoms, and that characteristics of autism are present in two-thirds of people with ADHD. “My clinical experience suggests it’s more than three-quarters in both directions,” adds Dr Eccles.

Online, the idea that autism and ADHD can coexist is so widely accepted that it has spawned its own label – “AuDHD” – and a groundswell of people who say they recognise its oxymoronic nature, perpetual internal war and rollercoaster of needs. There are tens of thousands of people in AuDHD self-help forums, and millions more watching AuDHD videos.

Some of those videos come from Samantha Stein, a British YouTuber. “The fact that you can have both [autism and ADHD] at the same time is kind of paradoxical in nature,” she admits. “You think: ‘How can you be extremely rigid and need routines and structure, but also be completely incapable of maintaining a routine and structure?’”

The 38-year-old started making videos on autism after her diagnosis in 2019, then began covering AuDHD after learning that she also had ADHD. “I realised that autistic adults – especially those who are diagnosed late in life – more often than not seem to have ADHD as well,” says Stein. Her first video on the subject, “5 signs you have ADHD and autism”, has now been viewed more than 2m times.

Some critics like to describe ADHD – and more recently autism – as a “fashionable” diagnosis, a misinformed excuse for life’s struggles. It’s almost inevitable that the new AuDHD label will cause a similar backlash. To see just how misguided this is, we must first understand both autism and ADHD.

Both are lifelong neurodevelopmental conditions that affect how people think, perceive the world and interact with others, according to Embracing Complexity, an umbrella group of organisations that research neurodiversity.

Autism and ADHD affect people on a spectrum of severity, both are legally recognised as disabilities, and neither are mental illnesses to be “cured”, although the knock-on effects can lead to mental illness. People who experience ways of thinking that diverge from those experienced by the majority of people are described as “neurodivergent”.

Autism spectrum disorder (ASD) is caused by multiple genetic factors that aren’t yet fully understood. Contrary to misconception, autism doesn’t equate to impaired intelligence, and only around half of people with autism also have a co-occurring intellectual disability. According to the National Autistic Society, autism is characterised by social challenges, repetitive behaviours, over- or under-sensitivity to surroundings and highly focused interests.

Autism is experienced in a multitude of ways. To empathise with the autistic trait of oversensitivity, for example, imagine that all your senses are amplified. The hum of your fridge is louder, the overhead lights are brighter, your itchy jumper is pricklier. It’s distracting while you’re trying to work, it’s draining to pretend it isn’t bothering you and you become increasingly stressed as a result.

“For me, eating in a canteen is like eating in a nightclub for a neurotypical person,” says Jill Corbyn, who is autistic and the director of support organisation Neurodiverse Connection. “It’s unpleasantly loud, it’s going to distract you from your food, it’s anxiety-inducing.” Additionally, some autistic people may find social situations exhausting or overwhelming, or feel incompetent when they’re unable to decipher the subtleties of interpersonal communication, 60% of which is non-verbal. Charli Clement, 23, explains that while a non-autistic person may rehearse parts of a conversation before a date or a job interview, her autism leads her to “script significantly” before even ordering a drink at a bar.

“I try to make sure I’m not doing something that will be perceived as ‘wrong’, so focusing on what the person is saying and what I should be replying is overwhelming,” she says.

Compounding the experience is the feeling many autistic people have that it isn’t “normal” to feel this way and that they must camouflage their discomfort to fit in with the pack. This “masking”, as it’s known, is exhausting, invalidating and can lead to burnout.

ADHD is also not fully understood. There’s evidence that the condition, involving an imbalance of neurotransmitters – including dopamine, in the brain – has both genetic and environmental causes. These chemical messengers are responsible for motivation, movement, planning, reward, memory, focus, alertness, impulse control and threat response, among others. People with untreated ADHD, whose reward pathways are therefore more dysregulated, can subsequently experience disordered moods, sleep, eating habits and dysfunction in almost every area of life.

Some people with ADHD are like pinballs of external chaos – of lost keys, missed appointments and cluttered homes. Others may appear inattentive, distracted by balls of chaotic thoughts into which they frequently retreat from the world to untangle.

ADHD affects people to different degrees. But many say their lives are marred by their brain’s misguided attempts to correct its chemical imbalances. They impulsively dopamine-spike with food, sex, drugs, booze, the internet, people, hobbies and novelty of all shades.

“I am a slave to my own brain and it’s tiring,” writes one anonymous person on an ADHD Reddit support group. Another asks: “Do you also feel like a slave to your desires?” She gives the examples of “chasing girls, gambling, chasing men, eating, hobby-hopping, extreme budgeting, falling in love [with] the wrong person, spending extravagantly”.

What frequently underpins the external and internal chaos, according to experts and many ADHDers alike, is a pervasive sense of deep shame and the quiet realisation that their potential in life is not being met.

When autism meets ADHD, it’s a curious form of alchemy, according to those who have both.

Sometimes the conditions are in conflict; at other times they’re symbiotic. There is no such thing as a perfect 50/50 split, explains Sadiq, and the brain is often “seesawing” between both conditions. This makes the presentation of AuDHD a distinctive condition in its own right, “completely different from pure ADHD or pure ASD”, he adds.

In his Ted Talk, “When Order and Anarchy Live Together”, Sadiq describes the dualities of the condition: “Silence v noise; structure v chaos; repetition v novelty; caution v risk-taking …”

Mattia Maurée, a non-binary composer and host of the AuDHD Flourishing podcast, discovered the AuDHD concept after following separate pieces of advice about autism and ADHD that “just weren’t working for me”. “It was like: ‘No, my life is still really, really hard,’” they tell me from Philadelphia.

AuDHD is uniquely “cyclical”, says Maurée, with big bursts of energy followed by a crash. “AuDHDers can also be incredibly creative and innovative, maybe because of that brain hyper-connectivity.”

Creativity is cited as the most positive AuDHD attribute by everyone I speak to, along with the subtle pairings of traits that “complement each other in a really nice way”, as Stein puts it. “ADHD gives me a love of novelty and a very creative side. And then autism allows me to focus on a topic that I’m really interested in. All of that allows me to be very self-directed.”

The paradoxes of AuDHD can camouflage each other or – on the surface at least – cancel each other out, which is why some AuDHDers experience missed or incorrect diagnoses.

In February, Sadiq saw a patient who had been referred to his NHS clinic for an ADHD diagnosis. He realised 15 minutes into the consultation that the patient was autistic. “If I had no lived experience of autism and ADHD I would have missed it completely,” he says. “I would have diagnosed either social anxiety or a personality disorder.”

In spite of his expertise, Sadiq is not formally qualified to make an autism diagnosis, and instead he had to refer the patient on to the autism service within the NHS trust. He believes that psychiatrists specialising in autism should also be trained in ADHD and vice versa, because otherwise “they’re going to be missing a lot”.

It’s not just the medical profession that needs more coordination. Charities such as ADHD UK and the National Autistic Society also work independently from one another. Legislation such as the government-backed The Buckland Review of Autism Employment, which recently called for employers to boost support for autistic people, scrutinises autism provisions but not ADHD ones. ADHD UK is one of many advocacy groups calling for the Autism Act, which legally compels the government to support autistic people, to be widened in scope to include other forms of neurodiversity.

Once a correct dual diagnosis is obtained, there are still complications. ADHD can be successfully managed with medication and behavioural coaching, but some autistic people react badly to this medication. Research indicates that stimulants are overall less tolerable for AuDHDers than they are for people with ADHD, according to the global research platform Embrace Autism, with one report finding that side-effects doubled in those with both conditions.

Another quirk of AuDHD treatment is that in some cases, it’s only after “quietening” someone’s ADHD symptoms that their autism traits come to the fore. This is often when people realise their autistic side for the first time, and it could explain why rates of self-reported autism closely follow those of ADHD.

The medical professionals I interviewed for this article were emphatic that ADHD medication cannot cause autism. Instead, Dr Eccles says: “It has just changed the balance of symptoms. The balance of masking has changed.”

The prevalence of autism was widely believed to be 1% until last year, when a first of its kind study published in the Lancet found the true rate to be more than double that, with at least 1.2 million autistic people in the UK. The prevalence of ADHD in UK adults is around 4%, according to ADHD UK, and assessment waiting lists for both conditions are increasing year on year, with waits of a decade in some parts of the country for ADHD assessment.

When naysayers argue that we are in the midst of an overdiagnosis epidemic, charities often point them to the statistics on suicide, and the fact that the ripple effects of ADHD and autism often lead to mental ill-health.

Autistic adults without a learning disability are far more likely to die by suicide. In 2022, researchers from Cambridge and Nottingham University, analysing coroners’ inquest records, concluded that a significant number of people who had died by suicide were likely autistic but undiagnosed. Adults with ADHD, meanwhile, are five times more likely to attempt suicide than their neurotypical peers.

Yet AuDHDers have been found to be at even greater risk of suicide than either those with only autism or ADHD, according to an academic study of more than 50,000 people.

For people like Clement, criticism about over-labelling is the least of her concerns. As a teenager she spent time in a psychiatric unit before the nature of her AuDHD was fully realised. “I’d already given myself labels,” she says. “I already thought that I was weird and broken. So having a label that actually made sense and encompassed my experience was so liberating.”

She now works part-time advising psychiatric hospitals on how to ensure their sensory environments are adequate for neurodiverse people.

Other AuDHDers give colourful analogies to describe the epiphany of diagnosis. Before the discovery, I’m told, it’s as if you are trying to fit in and be a horse rather than celebrating the fact that you’re a zebra. It’s like being trapped in a maze in the dark, then suddenly the lights are on and now there’s a way to navigate out.

Stein describes her life as “fundamentally walking parallel to, but never quite included in society”. Her diagnosis, however, “allowed me to look at my life through the lens of far more compassion – as a pretty good autistic person rather than a broken neurotypical person”.

“I think in some ways [AuDHD] can be a very beautiful thing,” she says.

“You just need the right support to be able to access those parts of you. And you need the label to know what the hell is going on in your brain.”

• In the UK and Ireland, Samaritans can be contacted on freephone 116 123, or email jo@samaritans.org or jo@samaritans.ie. In the US, you can call or text the National Suicide Prevention Lifeline on 988, chat on 988lifeline.org, or text HOME to 741741 to connect with a crisis counsellor. In Australia, the crisis support service Lifeline is 13 11 14. Other international helplines can be found at befrienders.org