Been doing some major searching on Guaif - as I sure do suffer from chronic sinus problems - I have a bunch of stuff printed off to show my doctor - including recent Action by FDA to allow 600 mg long acting tab without a prescription under the name of Mucinex - but I must issue a caution - apparently Guaif thins out mucous in the respitory system - lungs, sinuses, bronchial secretions but also improves (thins) cervical fluid - we have one daughter who now has 2 sons conceived while on the pill but during a time when she was taking Robitussin for a cold - in both instances she got pregnant.. So caution is advised in this area. But the research has been great. If you want really great medical info on this new drug application you can go directly to their website adamslaboratories or search googles. However. no reference was made to FM.