Readers sometime share the list of prescription from their MDs and ask for my comment. My usual comment are notes like this page, citing what we know from PubMed.
Valacyclovir and CFS
Studies have been done only on a subset of CFS that were EBV positive. One for FM and none for IBS.
Valacyclovir treatment of chronic fatigue in adolescents. (2014)
“. All patients were treated subsequently with valacyclovir, with 93% having a positive response. At the end of treatment, scores on fatigue self-assessment scales improved significantly (P < .001). Vigor subscale scores also improved significantly (P < .001). Some patients experienced complete resolution of symptoms. Although not every patient was tested, available laboratory testing revealed increased counts of natural killer (NK) cells and decreased human herpesvirus 6 (HHV-6) antibody titers in all patients who responded to valacyclovir. This article discusses the significance of infectious agents in the pathogenesis of psychiatric symptoms. The study’s data support an intriguing hypothesis that a portion of treatment-resistant depression in fact may be undiagnosed CFS or other chronic viral infection.”
Antibody to Epstein-Barr virus deoxyuridine triphosphate nucleotidohydrolase and deoxyribonucleotide polymerase in a chronic fatigue syndrome subset. (2012)
“There is prolonged elevated antibody level against the encoded proteins EBV dUTPase and EBV DNA polymerase in a subset of CFS patients, suggesting that this antibody panel could be used to identify these patients,”
Valacyclovir treatment in Epstein-Barr virus subset chronic fatigue syndrome: thirty-six months follow-up. (2007)
“Patients were given valacyclovir at 14.3 mg/kg every 6 hours… Sinus tachycardias decreased and abnormal cardiac wall motion improved. Serum antibody titers to EBV VCA IgM decreased. Patients resumed normal activities.”
A six-month trial of valacyclovir in the Epstein-Barr virus subset of chronic fatigue syndrome: improvement in left ventricular function. (2002)
” We concluded that the 16 CFS patients (included in both phases of this study) with EBV-persistent infection (EBV single-virus subset) are improved after 6 months of continuous pharmacokinetic dosing with valacyclovir. Nine CFS patients with EBV/human cytomegalovirus co-infection did not benefit from 6 months of similar treatment. Valacyclovir is not an effective anti-human cytomegalovirus antiviral drug.”
- “No effect of antiviral (valacyclovir) treatment in fibromyalgia: a double blind, randomized study.” 
EBV testing must be positive, CMV testing must be negative if you expect any benefit. Testing for EBV can result in some tests being negative (see below), so do not rely on a single negative test as being reliable.
EBV and Brain Inflammation
Note: Encephalitis is inflammation of the brain and that the traditional name for CFS is Myalgic Encephalomyelitis.
[Encephalitis due to the Epstein-Barr virus: a description of a clinical case and review of the literature]. 
“oral valaciclovir with clinical resolution and improvement of the liquoral parameters. Polymerase chain reaction in the cerebrospinal fluid was positive for EBV and negative for the other neurotropic viruses. In blood, the serology test for EBV with IgG was positive, while IgM and heterophile antibody tests [for EBV] were negative… EBV infection can give rise to acute disseminated encephalomyelitis or affect several locations in the central nervous system, especially the cerebellum.”
Ataxia and Encephalitis in a Young Adult with EBV Mononucleosis: A Case Report.
“The patient was clinically diagnosed with EBV-associated cerebellitis and encephalitis, displaying neurological and psychiatric impairment commonly seen in postconcussion syndrome. MRI showed no acute changes. She was started on valacyclovir and a prednisone taper, recovering by the end of twelve weeks.”
[Effect of the Epstein-Barr virus on the nervous system]. 
“it has been shown that this infection can be accompanied by acute and chronic affections of the central and peripheral nervous system. The pathogenesis of chronic EBV-infection involves autoimmune disorders, neurosensitization, a hazard of an injury to the muscular tissue”
Alternative EBV treatment
- Bosewellia “showed potent inhibitory effects on EBV-EA induction” 
- The effect of curcumin on human B-cell immortalization by Epstein-Barr virus.  “A strongly promote in vitro B-cell immortalization with EBV … and curcumin, an extract of a common spice is an effective inhibitor of this process;”
- “Seven rhizomes were found to possess inhibitory activity towards EBV activation, induced by TPA; they are:Curcuma domestica, C. xanthorrhiza, Kaempferia galanga, Zingiber cassumunar, Z. officinale, Z. officinale (red variety), and Z. zerumbet.” 
- White Mugwort (Artemisia lactiflora) ” In addition, AL-1 strongly inhibited tumor promoter-induced Epstein-Barr virus (EBV) activation” 
This is effective for a subset of CFS patients that are positive for only one Herpes virus, EBV. Multiple virus or no virus are unlikely to benefit according to the studies.