BACKGROUND: The self-presentation of psychiatric disorders on social media has generated a countermovement of sceptics who believe some are "faked" for reasons of self-promotion. This has been the focus of a Reddit forum called r/FakeDisorderCringe, an active community with almost 300,000 members. Given the influence of online discourse on public attitudes, understanding which diagnoses attract the most scepticism and what conversational themes surround these concerns is important in understanding lay controversies in mental health. METHODS: We used topic modelling using Latent Dirichlet allocation, a natural language processing technique, to analyse over 850,000 forum posts from August 2020 to December 2022 to identify the main topics. RESULTS: Topic modelling identified 15 topics, of which several identified specific disorders: anxiety, depression, OCD (obsessive-compulsive disorder), ADHD (attention deficit and hyperactivity disorder), autism, Tourette's syndrome and tics, and DID (dissociative identity disorder). Other topics included faking and attention-seeking, school and childhood context, trauma and abuse, validity of self-diagnosis, medical research evidence, gender identity and sexuality, profanity, and sarcasm-laden commentary. CONCLUSIONS: This scepticism does not mirror previously identified public doubts about the legitimacy of specific diagnoses and seems to more commonly focus on conditions that have seen a recent increase in diagnosis in young people.