Attention deficit hyperactivity disorder (ADHD) was previously seen as a childhood disorder that improves with age; however, research has shown that ADHD continues into adulthood affecting all areas of life. ADHD has been found to frequently co-occur with many mental health diagnoses, changing their clinical presentation. Yet the absence of an ADHD diagnosis has been seen to exert even more influence on mental health than ADHD symptoms alone. Despite an increase in the recognition of adult ADHD, individuals describe many challenges faced in receiving a diagnosis including stigma, misdiagnoses, and a lack of support. This thesis aimed to improve awareness and understanding of adult ADHD, its impact on mental health and the experience of seeking a diagnosis. With the hope that this can in turn support the recognition, diagnosis and treatment of ADHD in adulthood.

Part A of this thesis presents a systematic review and meta-analysis exploring ADHD and anxiety symptomology and severity. Analysis found that individuals with ADHD score higher on measures of anxiety than individuals without ADHD with a moderate effect. This effect is moderated by the type of comparison group used, with the difference seen being larger when individuals with ADHD are compared to a non-clinical population than a clinical population.

Part B presents a qualitative analysis of adults’ experiences of seeking an ADHD diagnostic assessment in the UK, from the perspective of those who are currently on a waiting list. Six overarching themes were developed reflecting participants’ journeys. These cover what led them to seek a diagnosis, how they have found the process and what changes they feel are needed to improve this experience for adults in the future.