Trial of Mirtazapine for Depression in IBD

Inclusion Criteria: 1. Established diagnosis of Crohn's disease (CD) or ulcerative colitis (UC) according to clinical notes 2. Fulfil Diagnostic and Statistical Manual of Mental Disorders (5th Edition) (DSM-5) criteria for current single or recurrent episodes of major depressive disorder of at least moderate severity but without psychotic features, as defined by the Mini Neuropsychiatric Interview 3. Aged 18 years or over 4. Use of contraception if female and of childbearing age. Female participants of childbearing age will require a negative serum/urine pregnancy test before starting the study and will also need to agree to use an acceptable form of contraception throughout the intervention period, e.g. oral contraceptive pill, long-acting reversible contraceptive 5. Able to provide written informed consent to enter the trial Exclusion Criteria: 1. Diagnosis of drug or alcohol dependence syndrome according to the General Practitioner (GP) summary care record or yes in answer to the question "have you ever been diagnosed with drug or alcohol dependence?" 2. Diagnosis of any personality disorder according to the GP summary care record or yes in answer to the question "have you ever been diagnosed with a personality disorder?" 3. Diagnosis of any dementia according to the GP summary care record or yes in answer to the question "have you ever been diagnosed with any form of dementia?" 4. Diagnosis of psychosis or schizophrenia according to the GP summary care record or yes in answer to the question "have you ever been diagnosed with psychosis or schizophrenia?" 5. Diagnosis of bipolar disorder according to the GP summary care record or yes in answer to the question "have you ever been diagnosed with bipolar disorder?" 6. Current active suicidal ideation on clinical assessment according to clinical judgment by a study consultant psychiatrist 7. Current treatment with mirtazapine, mianserin, trazodone or a monoamine oxidase inhibitor 8. Contraindications to the administration of mirtazapine, as per the current summary of product characteristics 9. Known allergy to mirtazapine or mianserin according to patient report or clinical notes 10. Non-registration with a GP or failure to consent to sharing of the GP summary care record and any psychiatric assessments held 11. Currently enrolled in another drug trial or psychological therapy trial 12. Currently hospitalised for the treatment of IBD 13. Currently being prescribed a course of budesonide or reducing course of prednisolone for IBD 14. Planned change in IBD treatment within the next 12 weeks according to clinical notes or answer yes to the question: "are you expecting any change in your IBD treatment to take place in the next 12 weeks?"