 One hypothesis behind low-dose naltrexone's mechanism of action is that inhibiting opioid receptors at low doses might cause the body to increase production of endorphins and upregulate the immune system; it may also antagonize Toll-like receptor 4 that are found on macrophages, including microglia, and any reported anti-inflammatory effects might be due to that.These apparent effects have not been seen at standard doses or in major trials.Researchers have also examined "ultra-low-doses" of naltrexone at microgram, nanogram, and picogram doses, that are co-administered with opioid analgesics with the goal of increasing pain relief and reducing side effects.