Globally, more than 300 million people suffer from depression; it is now the leading cause of disability worldwide. Major depressive disorder affects up to one in six New Zealanders in their lifetime but current treatments are ineffective in up to 30% of patients. Recent research suggests that brain inflammation may play a role in depression; however, at this time, there are no diagnosis or treatment methods relating to inflammation in depression. This study will test whether low-dose naltrexone (LDN) – a drug that is reported to have anti-inflammatory effects in the brain – in addition to conventional medications will treat symptoms of depression more effectively. Blood samples and brain scans will also be used to measure markers of inflammation. There is potential to identify blood- or brain-based biomarkers that can help us further understand the role of inflammation in depression and identify individuals who might respond better different medications.