It is true that probiotics (the beneficial bacteria in our gut) produce important substances, including neurotransmitters such as serotonin. Serotonin is often referred to as the happy hormone because it is involved in regulating our mood. Low levels of serotonin are associated with depression and anxiety. For that reason, drugs and products that work to elevate serotonin levels are found to be helpful in treating these conditions. There is mounting research to suggest that probiotics may play a role in improving mood. More research is need to confirm whether certain probiotics can be beneficial in treating depression, but for now it appears that this may be one side benefit of taking probiotics.