*2.2. Procedure*

Participants were randomly assigned to the following groups: experimental (ER-GYPHILUS Plus (Laboratorios NUTERGIA S.L., San Sebastián, Spain)), which contained *Lactobacillus rhamnosus GG*, *L. paracasei*, *L. acidophilus* and *Bifidobacterium bifidus* (revivification of 6 million germs per capsule, 4 capsules per day, *n* = 16), or placebo (*n* = 15). The placebo capsules were composed of cellulose and provided by Complementos Fitonutricionales S.L. (Spain). The evaluation was performed both before the treatment (baseline) and after 8 weeks of treatment (post-intervention). More information about the procedure can be found in the first part of the study [31]. The duration of treatment was selected according to similar, previous research [33,34].

The selected probiotic species have been used previously to improve functions related to the gut–brain axis [33,35,36] and are therefore expected to be capable of attenuating the cognitive and emotional changes caused by FMS.

## *2.3. Outcome Measures*

## 2.3.1. Demographic Measures

All participants provided the following demographic and clinical information: gender, age, FM diagnosis onset, years of formal education and body mass index (BMI). The BMI index was calculated by dividing the weight by the square of the height.
