Depression is a common and disabling illness that is an important global public health concern. It is now the leading cause of ill health and disability worldwide (WHO, 2017). At least a third of people do not respond to the standard treatments for depression, namely antidepressants and cognitive behavioural therapy (CBT) (Rush et al 2006). Higher levels of serum lipids have been reported to be significantly associated with depression in recent large studies (Oh & Kim 2017). Simvastatin is from a group of drugs called HMG CoA reductase inhibitors, or statins. It reduces levels of low-density lipoprotein (LDL) and triglycerides in the blood, while increasing levels of high-density lipoprotein (HDL). This is a randomised controlled trial with 120 people with treatment-resistant depression with the aim of determining whether the addition of simvastatin (20mg daily) to treatment as usual (TAU) for 12 weeks leads to an improvement in depressive symptom compared with placebo added to TAU.