Metronidazole/Flagyl is reported to have 75% of CFS patients improving according to some surveys. I was asked what is the best dosage…  that is for your MD to determine.

See this post to understand how it may be helping CFS — probably not how you think!.

MDs are busy people and often do not have time to check the recent literature. I’m busy too, but made time for this. My results (for educational purposes) are below

• Metronidazole relieves symptoms in irritable bowel syndrome: the confusion with so-called ‘chronic amebiasis'[1997].
  • metronidazole (400 mg tid X 10 days)
• Systematic review and meta-analysis: triple therapy combining a proton-pump inhibitor, amoxicillin and metronidazole for Helicobacter pylori first-line treatment[2016].
  • “Use of 14 day, thrice daily and high-metronidazole-dose PAM treatments markedly increased the cure rate.”
• Secnidazole Treatment of Bacterial Vaginosis: A Randomized Controlled Trial [2017].
  • Secnidzaole is a variation of metronidazole. 2 grams/day performed better than 1 gram/day
• Improving malodour management in advanced cancer: a 10-year retrospective study of topical, oral and maintenance metronidazole.
  • “High-risk patients should start with 400 mg thrice daily ×7 days and continue 200 mg once daily.”
• Randomized, double-blind, comparative study of oral metronidazole and tinidazole in treatment of bacterial vaginosis [2016].
  • “tablet metronidazole 500 mg twice daily for 5 days”
• Variability in Antibiotic Regimens for Surgical Necrotizing Enterocolitis Highlights the Need for New Guidelines.
  • ” The most common pre-operative regimen was … metronidazole for 14 d.  The most common post-operative regimen was … and metronidazole for two days. No regimen or duration proved superior.”

From Drugs.com with more details on Medscape.

Do any MDs use it for CFS?

There are no dosage studies or guidance for CFS/ME. From the above, I would conclude the following as a reasonable plan:

• 400-500 mg three times a day for 14 days, followed by 200 mg/daily for at least 2 months.

NOTE: this applies to other antibiotics also
“Metronidazole can have cerebellar toxicity that manifests clinically with varying degrees of limb and gait ataxia and dysarthria (Table 2). Symptoms are accompanied by characteristic T2 high signal lesions on brain MRI in the cerebellum and brainstem. Neurotoxicity was seen after prolonged use of metronidazole with clinical symptoms resolving within 3–7 days of discontinuation of the medication, while follow-up MRIs also show resolution of cerebellar lesions [111]. While the precise mechanism for neurotoxicity is not entirely clear, one hypothesis is that it occurs via axonal swelling secondary to metronidazole-induced vasogenic oedema [112]. Peripheral neuropathy is another recognized potentially neurotoxic effect with use of metronidazole. One report describes a 53-year-old who developed encephalopathy, dysarthria, ataxia and a length-dependent peripheral neuropathy in the context of prolonged metronidazole therapy (a cumulative dose of 146 g over 88 days). Multiple skin biopsies confirmed evidence of a small fibre sensory neuropathy [113]. In another case reporting the rapid development of peripheral neuropathy related to metronidazole with electrophysiologic studies demonstrating prolonged distal motor latencies, mildly decreased compound muscle action potential and decreased sensory nerve action potentials involving the posterior tibial and peroneal nerves to varying degrees were noted [114]. Optic neuropathy, as well as autonomic neuropathy, have also been described in association with metronidazole use [115, 116]. Other neurological adverse effects ascribed to metronidazole include dizziness, headache and confusion [117].” 2011

Again, not medical advise, just a summary of the literature and conclusions from the literature for educational purposes. Applicability to individuals need to be done by a qualified medical professional who knows your medical history.