1. Suggested design of study.

Methods	Allocation: the randomisation process should be clearly described. Trialists should take every precaution to minimise the effect of biases by using blinded or independent raters. Intention‐to‐treat analysis is preferable: trialists should describe from which groups withdrawals came, why they occurred and what was their outcome. Duration: 6 months minimum. Setting: in an outpatient psychiatric clinic, where patients with delusional disorder are most likely to be encountered.
Participants	Diagnosis: people with delusional disorder. Age: all ages. Sex: men and women. N = 300*. History: to include the full range of severity of this disorder, from people who have been hospitalised in the past to never.
Interventions	1. Treatment as usual + randomised to intervention (e.g. 1st generation antipsychotic medication, at low dose). N = 150. 2. Standard treatment as usual, including patient's usual antipsychotic medication (at current dose). N = 150. The above trial could be amended to address psychological therapy as an intervention and patients could be accordingly randomised to this in addition to treatment as usual, compared to usual care.
Outcomes	Real world clinical outcomes, e.g. global improvement, use of scales such as CGI, mental state. Service outcomes: readmission, frequency of clinic appointments. Loss to follow‐up. Functioning: including employment. Serious events: any, list. Satisfaction. Quality of life. Economic outcomes.
Notes	* Size of study with sufficient power to highlight ˜20% difference between groups for primary outcome.

CGI: Clinical Global Impression Scale