Only a few studies have looked at the effectiveness of CBD in treating fibromyalgia. Researchers cannot decisively say why the compound appears to reduce some fibromyalgia symptoms, but there are some strong theories that are still being tested in research. They also do not understand why it works in some people and not in others. The pain-relieving effects of CBD may be explained by the ways that this substance affects the brain. It may interrupt the nerve pathways that send signals of pain between the brain and the body. There is also an anti-inflammatory action of CBD that would reduce heat and swelling around injury or disease. This, in turn. would reduce pain. There is also debate about whether CBD is more effective when used alone or as medical marijuana. A combination of other chemicals in the plant may intensify the positive effects of CBD and provide additional benefits. One 2006 study determined that CBD worked best in combination with THC, but little research has followed. A 2016 study suggested that a lack of endocannabinoids — neurotransmitters that bind to cannabinoid receptors — may be at the root of chronic pain syndromes, including migraines and fibromyalgia. Taking CBD may correct this deficiency, explaining the compound's success in alleviating chronic pain.