What you need to know about intrusive thoughts and OCD

Having 'intrusive thoughts' is a catch-all term for any thoughts that are unwelcome: graphic sexual or violent thoughts, thoughts of doing, saying or seeing something taboo, or generally unwelcome mental images or messages.

Intrusive thoughts in themselves are not a condition or symptom of anything. One 2014 study suggested that 94% of people thought they had experienced them. They're unpleasant and uninvited, but in almost all cases they come and go without consequence.

But in people with obsessive-compulsive disorder, the experience is starkly different. OCD is a chronic mental health condition, and (though perhaps underdiagnosed) it's a lot less common than intrusive thoughts. Current estimates are that 1.2% of the population is affected by OCD over a 12-month period (not including their loved ones who might be impacted by supporting them). That means that at any one time, more than 750,000 people in the UK are living with OCD.

Living with OCD and intrusive thoughts

An OCD diagnosis comes from a combination of two symptoms: obsessive thoughts and compulsive behaviour. When a person with OCD experiences intrusive thoughts, they then have an urge to do something to cope with how the thoughts make them feel. The urge might be to do a verbal or physical task (asking for reassurance, or hand-washing) but it might also be going through a thought process (exhaustively thinking over an event).

David Adam's memoir of his experience of OCD, The Man Who Couldn't Stop, became a Sunday Times bestseller after its publication in 2016.

In it, David tells the story of how he developed OCD. In the summer after his first year of university, he began to have intrusive thoughts about touching objects that might have come into contact with HIV-laden blood. After these thoughts, David began checking and re-checking objects he'd touched as a way of trying to reassure himself. (Note: HIV cannot be transmitted simply by touching a surface. In the UK most infections are caused by having unprotected sex with an HIV-positive person who is not using effective treatment.)

Initially, David says, therapy he received for OCD was not much help. But after he went back into treatment, he received cognitive behavioural therapy (CBT) and was prescribed medication to treat the OCD.

"What they taught me in CBT is that intrusive thoughts are really common, and almost everybody has them," David says. "But the way you respond to these thoughts and process them is where OCD pathology lies."

What causes OCD?

OCD usually appears in late adolescence (common for men) or early 20s (common for women), but can appear in children. OCD UK estimates that a quarter of cases start by the age of 14. Onset after the age of 35 years is unusual, but does occur - for example, in postnatal OCD.

Unlike diseases like type 2 diabetes or PTSD, OCD can't be said to have a particular cause. Risk of developing OCD is influenced by both a genetic predisposition to anxiety, and environmental factors like stress or trauma, but isn't directly caused by either factor.

For David, OCD appeared out of nowhere.

"It was the summer after my first year of university and I was really happy," he recalls. "I was having a really good time and looking forward to getting back to university. I can't place an incident, stressor or traumatic experience which was triggered at that time.

"As I say in the book, that first intrusive thought was so out of place, it was as if a snowflake had fallen from the sky in the middle of summer. It was so weird and out of place."

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Compulsive behaviour

Compulsive behaviour in OCD can be so subtle as to appear invisible, even to people who have it. Compulsions might be physical (cleaning or touching) but they might also be mental (private thought, tasks or rituals). Compulsions might also include 'checking' or asking for reassurance.

Compulsions feel 'addictive' because they temporarily relieve anxiety. But however well they appear to work at the time, compulsions do not resolve anxiety. People might seek help for OCD when the urge to check or do a certain action gets in the way of their work or school work, or worries their friends or family. NICE advises that people with OCD receive therapeutic interventions, and some patients also find that medication such as antidepressants also decreases symptoms.

A couple of years ago, there was a debate around whether a type of OCD exists that is characterised as 'purely' an obsession with intrusive thoughts, without compulsions - commonly referred to as 'pure O'.

But clinicians are sceptical, and OCD-UK has published a position statement clarifying that the charity doesn't consider it to be a form of OCD. In its 'mythbuster' OCD-UK clarifies that some compulsive behaviours in OCD might be more subtle than others - for example, mental rituals, checking/reassurance, or avoiding people or places. Nevertheless, 'invisible' compulsions still dog the person's intrusive thoughts.

Living with intrusive thoughts

David still considers himself to have OCD, but his symptoms are well managed. He lives in London and works as a science writer and editor for national newspapers.

"When people ask me how I feel now, I always say that I have good days and bad days, and that's a lot better than only having bad days," he explains. "I still have weird thoughts about HIV and AIDS, but I am better at resisting the compulsion 'loop', which is what drives the OCD."

Dr Lynne Drummond is a psychiatrist with 35 years' experience, and worked most recently as the lead clinician for the National OCD/BDD Service for the UK.

In Dr Drummond's book, Obsessive Compulsive Disorders, she explains that when people with OCD tell friends and family what they're concerned about, their worries might seem overblown or hard to understand. At first, people might be tempted to use humour or play down the worries - missing the emotional reality that the person with OCD is facing.

"These thoughts are profoundly disturbing to the individual with OCD," Drummond writes, and are therefore extremely serious in their mind. Although they may realise their thoughts are either irrational or exaggerated, the fear accompanying these thoughts is very real and they are unable to 'snap out of it'."

She urges people to seek further help for symptoms that don't change after CBT or counselling.

"Both I and my colleagues in psychiatry notice that we rarely see people with OCD, despite it affecting 1-3% of the population," she says. "I think people go to Improving Access to Psychological Therapies (IAPT) services, and if they don't get better after that they tend to believe there's nothing else for them.

"OCD can be very serious. It can impact on the whole family and ruin lives. But there are effective treatments out there."