When it's more than a habit, and other hard cases

When the checking has taken over. If it takes up a lot of your time, causes real distress, or interferes with work, relationships or leaving the house, the NHS describes that as obsessive compulsive disorder and says it's unlikely to get better without treatment. The main treatment is a talking therapy, usually cognitive behavioural therapy with exposure and response prevention, which involves facing the doubt without doing the compulsion, starting with the easiest situations. It's the same principle as this book, delivered by someone trained, with support, and it works for many people. You can see a GP or refer yourself directly to an NHS talking therapies service. Medicine can help too, and that's a conversation with a GP. Nothing here should delay that.

When the checking is about a baby. Checking a baby's breathing repeatedly, with unwanted and frightening thoughts, is something the NHS specifically describes. Speak to a GP or health visitor. It's common, it's treatable, and it isn't a sign of anything about you as a parent.

When you've actually been leaving things. If the hob really has been left on, or the door really has been left open, more than once recently, do the once from chapter 17 with more care, and mention the forgetting to a GP if it's new or spreading. A habit of rechecking is one thing. A memory that's changed is another, and it deserves a look.

When your partner asks you to check for them. Agree a sentence together. Something like "I'm not going to answer that, because you know the answer and answering makes it worse." Say it kindly and mean it. The first few times will be hard for both of you. It's the most useful thing you can do for someone in the loop.

When you're the one who's been asking. Tell your partner what you've learned and ask them to stop answering. Give them the sentence above. Then when you ask anyway, and you will, they can use it, and you can be grateful rather than hurt.

When you're away from home. Hotel rooms, other people's houses and hire cars all offer new things to check. Do the once, say the sentence, and treat the rest exactly as at home. The doubt is portable and so is the method.

When it's a high-stakes email. The contract, the resignation, the message to the ex. Read it more than twice before sending if it warrants it. Sleep on it if it warrants that. Then send, and close the folder. The stakes raise the care that goes into the once. They don't change what happens after.

When the doubt won't fade. Occasionally it holds for longer than twenty minutes. Let it. Do the next thing anyway. The doubt isn't dangerous, and it's not made smaller by attention. If doubt of this kind is with you most of the day, most days, that's a matter for a GP, whether or not the checking has taken over.

When someone else's habit is worse than yours. You may live with a checker. The kindest thing is not to join in and not to mock. Don't check for them, don't answer, and if it's taking over their life, suggest, once, that they talk to a GP.

When you slip. You'll go back to the door, or open the sent folder, or ask. Note it, and note that the door was locked, and do the once tomorrow. One recheck doesn't restart anything. What restarts the habit is deciding that the recheck proved something.