One of the ways that CBD is thought to work for anxiety and depression is by increasing the production of serotonin. Serotonin syndrome (SS) occurs when you take medications that cause high levels of the chemical serotonin to accumulate in your body. Serotonin is involved with the body’s nerve cells and brain function. Too much serotonin causes symptoms that can range from mild (shivering and diarrhea) to severe (muscle rigidity, fever and seizures). Studies show that SS is uncommon and may occur only with high CBD concentrations, at about 20mg/kg.