Low serotonin might not cause depression, but why do SSRIs still work?

Depression is a highly prevalent mental illness. Treatment options for depression are individualized and may include the use of medications and therapy. Researchers have theorized that low serotonin levels cause depression. Data from a recent systematic umbrella review found little evidence linking serotonin levels with depression. Clinical depression is one of the most common mental illnesses, impacting millions of people worldwide. While several factors contribute to depression, one common idea is that it is related to chemical imbalances in the brain, particularly low levels of the chemical serotonin.