Hi @marenew, I have no experience with Mexiletine but thought I would share a few references that mention the drug with you while you wait for other members with experience to share what they've learned.

-- Neuropathic Pain: Principles of Diagnosis and Treatment:
https://www.mayoclinicproceedings.org/article/S0025-6196(15)00105-6/fulltext
-- Office approach to small fiber neuropathy:
https://www.ccjm.org/content/85/10/801
-- Small fiber neuropathy as a part of fibromyalgia or a separate diagnosis?
https://www.openaccessjournals.com/articles/small-fiber-neuropathy-as-a-part-of-fibromyalgia-or-a-separate-diagnosis-12822.html