Reishi Mushroom Extract for Fatigue and/or Arthralgias/Myalgias in Patients With Breast Cancer on Aromatase Inhibitors

Inclusion Criteria:

* Age ≥ 18 years
* History of breast cancer, estrogen receptor positive (ER+), Her 2 positive or negative
* Fatigue ≥ 4/10
* Currently post-menopausal (as defined by National Comprehensive Cancer Network (version 4.2024), taking any aromatase inhibitor in the curative setting and planning to be on such for at least 8 weeks after registration. \[Patients on concurrent ovarian suppression (such as with leuprolide acetate, goserelin) are allowed\]; CDK 4/6 inhibitors abemaciclib, ribociclib ARE allowed
* Prior treatment: last chemotherapy ≥ 90 days prior to randomization (if treated with chemotherapy)
* On a stable dose of pain medications if pain medications are being regularly used. (i.e., no change in dosage in the past 30 days)
* If on supplements, must be on stable dose with no plan to change; not on or planning any acupuncture or other specific supportive modalities for fatigue or AI arthralgias
* Eastern Cooperative Oncology Group (ECOG) performance status (PS) 0, 1 or 2
* White blood cell count (WBC) ≥ 3,000/mm\^3 (obtained ≤ 30 days prior to randomization)
* Hemoglobin ≥ 10 g/dL (obtained ≤ 30 days prior to randomization)
* Platelet count ≥ 100,000/mm\^3 (obtained ≤ 30 days prior to randomization)
* Total bilirubin ≤ 1.5 x upper limit of normal (ULN) (obtained ≤ 30 days prior to randomization)
* Alanine aminotransferase (ALT) or aspartate transaminase (AST) ≤ 1.2 x ULN (obtained ≤ 30 days prior to randomization)
* Prothrombin time (PT)/activated partial thromboplastin time (aPTT) ≤ 1.5 x ULN (obtained ≤ 30 days prior to randomization)
* Negative pregnancy test done ≤ 7 days prior to registration, for persons on concurrent ovarian suppression only
* Provide informed consent
* Ability to complete questionnaires
* Willing to return to enrolling institution during the active monitoring phase of the study
* Patients who have had a recent surgery or procedure should be healed and cleared by their clinician and/or surgeon per local standards, prior to registration

Exclusion Criteria:

* Other known uncontrolled medical conditions causing fatigue such as untreated thyroid disease, depression, fibromyalgia, chronic fatigue syndrome, infection, autoimmune disease, or active/untreated hepatitis
* Allergy to mushrooms
* On anticoagulation medication or aspirin or having a known bleeding disorder
* On any specific medication for fatigue (e.g., methylphenidate)
* Metastatic cancer diagnosis (history of nodal metastases is allowed)
* Chronic steroid use, unless on physiologic replacement doses
* Current use of any medical mushrooms
* On medications for diabetes
* History of symptomatic hypotension
* Taking CYP3A4, CYP2D6 sensitive substrates which can be located at the following link:

https://www.fda.gov/drugs/drug-interactions-labeling/healthcare-professionals-fdas-examples-drugs-interact-cyp-enzymes-and-transporter-systems

* Drugs which exhibit either \>20% inhibition or \>20% induction of CYP2E1 in vivo, such as: Acetaminophen, Dapsone, Enflurane, Halothane, Isoflurane, \& Theophylline
* Taking olaparib
* Any of the following because this study involves an agent that has unknown genotoxic, mutagenic and teratogenic effects:

  * Pregnant persons
  * Nursing persons
  * Persons on concurrent ovarian suppression who are unwilling to employ adequate contraception (e.g., hormonal methods, barrier methods, intrauterine device, abstinence)
* Planned surgery or procedure during time on study and ≤ 14 days after last dose, due to bleeding risks