Has mucinex (guaifenesin) shown any testosterone lowering as a side effect.?

Very Doubtful: The only possible correlation with testosterone would be through dehydration and coincidental liver disease. An on-line study showed no reports of elevated sex hormone binding globulin (linked to testosterone levels) and mucinex (guaifenesin). If you had a hypersensitvity reaction that involved the liver testosterone could go down.

Not to my knowledge: If there is any effect of Mucinex (guaifenesin) on testosterone levels or the androgen axis, it is not felt to be strong (or widely known).