Navigating Your Way Through Brain Fog. Brain fog is a symptom of several chronic pain problems, notably fibromyalgia, from where it gets the commonly used name of fibro fog. It can accompany a number of medical conditions, including vascular problems, diabetes, and as a result of contact with a toxic substance. It is also a side effect of chemotherapy treatment used for cancer patients. Whatever the health issue that gives rise to brain fog, the effects are broadly similar. In combination with the chronic fatigue that is a frequent symptom of pain conditions, it can have a significant impact on your quality of life.