 This 12 month parallel group, multi-centre, patient randomised, pragmatic, open label trial is comparing the clinical and cost-effectiveness of the decision to prescribe lithium versus quetiapine add-on treatment to antidepressant medication. There will be two parallel groups: 1 Quetiapine add-on to existing antidepressant medication; 2 Lithium add-on to existing antidepressant medication. 276 patients will be randomised 1:1 at baseline to the decision to prescribe either lithium or quetiapine, and treatment will then be undertaken by clinicians on a real world basis. The acute phase of treatment 8 weeks post-randomisation will be conducted by trial clinicians with a decision as to then continue treatment as per standard care thereafter. All patients, regardless of their treatment status, will be followed up in the trial for one year. This is a superiority design whereby we hypothesise that quetiapine will be superior to lithium in terms of time to treatment discontinuation and average symptom burden QID-SR over 12 months. 