BDNF and fibromyalgia: One study found higher serum τ and Aβ in patients with fibromyalgia compared with controls [73]; and (of the remaining 11 studies that investigated serum BDNF levels in serum) one found no difference in BDNF levels between groups [74]; one found lower BDNF levels in patients with chronic pain [75]; and nine found higher levels of BDNF in chronic pain groups compared with control groups.