Pain and stiffness in the muscles, which is often associated with disturbance of sleep. Fibromyalgia is most common in women aged between 40 and 60 years. The condition is also known as chronic widespread pain (CWP) and was formerly referred to as fibrositis. Medical investigation usually fails to reveal any underlying cause and, for this reason, some doctors do not recognize fibromyalgia as a medical condition. However, the lives of affected people are often severely disrupted. The muscles most commonly affected are those in the neck, shoulders, chest, back, buttocks, and knees. There is usually no restriction of movement. Attacks are generally worse in cold, damp weather. Exhaustion and disturbed sleep are common. Use of analgesic drugs (painkillers), hot baths, and massage usually relieves pain and stiffness. Antidepressant drugs, such as amitriptyline, are often effective in improving sleep and relieving pain.

Between 8 and 10% of adults report suffering from chronic diffuse musculoskeletal pain, and about half of these satisfy the classification criteria for fibromyalgia. The aetiology of fibromyalgia is unknown, but recent data indicate that psychological distress is a strong predictor of the development of this condition. Although the pain is felt primarily in the muscles, the muscles show no histological or metabolic abnormalities other than those associated with physical deconditioning. Management that includes patient education, cognitive-behavioural approaches, regular aerobic training, and low-dose tricyclic agents generally provides benefit only to few patients. 

It is possible, as has been suggested, that fibromyalgia (for example) may represent a central disorder of pain perception, perhaps associated with altered levels of substance P or nerve growth factor.