• Self rated symptoms of depression using the Beck Depression Inventory [ Time Frame: Repeated measures up to 6 months after initiation of folic acid/placebo ] [ Designated as safety issue: No ]
  

• Clinician rated depression using the Montgomery−Asberg Depression Rating Scale [ Time Frame: Repeated measures up to 6 months after initiation of folic acid/placebo ] [ Designated as safety issue: No ]
  
• Clinician rated symptom severity using the Clinical Global Impression scale [ Time Frame: Repeated measures up to 6 months after initiation of folic acid/placebo ] [ Designated as safety issue: No ]
  
• Health status using the SF12 [ Time Frame: Repeated measures up to 6 months after initiation of folic acid/placebo ] [ Designated as safety issue: No ]
  
• Adverse events [ Time Frame: Repeated measures up to 6 months after initiation of folic acid/placebo ] [ Designated as safety issue: Yes ]
  
• Cost Utility using the EuroQol, resource use questionnaire and medication history [ Time Frame: Repeated measures up to 6 months after initiation of folic acid/placebo ] [ Designated as safety issue: No ]
  
• Folate status [ Time Frame: Baseline, 3 months and 6 months ] [ Designated as safety issue: Yes ]
  
• Homocysteine Status [ Time Frame: Baseline, 3 months and 6 months ] [ Designated as safety issue: No ]
  
• The interaction between any of the genetic polymorphisms in the folate pathway that predicts the severity of depression, response to antidepressants, and the response to folate supplementation [ Time Frame: Baseline only ] [ Designated as safety issue: No ]
  
• Compliance - using the number of tablets remaining at each follow up, dispensing records for folic acid or placebo, dates of repeat prescriptions, Morisky questionnaire, red cell folate and homocysteine levels [ Time Frame: 12 weeks and 6 months ] [ Designated as safety issue: No ]
  

Clinical depression is common, debilitating and treatable; one in four people experience it during their lives. The majority of sufferers are treated in primary care and only half respond well to active treatment. Evidence suggests that folate may be a useful adjunct to antidepressant treatment: 1) patients with depression often have a functional folate deficiency; 2) the severity of such deficiency, indicated by elevated homocysteine, correlates with depression severity, 3) low folate is associated with poor antidepressant response, and 4) folate is required for the synthesis of neurotransmitters implicated in the pathogenesis and treatment of depression.

The primary objective of this multi-centred placebo-controlled randomised trial is to estimate the effect of folate augmentation in new or continuing treatment of depressive disorder in primary and secondary care. Secondary objectives are to evaluate the cost-effectiveness of folate augmentation of antidepressant treatment, investigate how the response to antidepressant treatment depends on genetic polymorphisms relevant to folate metabolism and antidepressant response, and explore whether baseline folate status can predict response to antidepressant treatment.

Comparisons: Eligible patients with moderate to severe depression will be randomised to receive 5mg of folic acid or placebo as an adjunct to their antidepressant treatment.