Improvement was clear after 15 days and continued for up 60 days, after which it generally leveled off. Side effects were generally mild and transient. No one really knows. It is thought that low serotonin levels may be associated with greater pain sensitivity. Some evidence suggests that increasing serotonergic function using drugs like the SSRIs or 5-HTP may, therefore, raise pain thresholds.
Lack of sleep is often an important factor in fibromyalgia.
Thus, improving sleep patterns may also contribute to the feelings of relief.