Medications know to cause bone marrow suppression (e.g., myelosuppressive antineoplastic agents) may result in a blunted or impeded response to methylcobalamin, vitamin B12 therapy
High Ferritin Iron Overload/Toxicity As a side rabbit hole, while I had been struggling with low ferritin that was discovered in 2011 and resolved in 2017 (it also flared up in 2020, then again in 2022), ironically, my husband had iron overload that was diagnosed in 2018, and it resolved in 2021 due to some extreme lifestyle changes
Kidney failure and liver damage: Untill now, studies in patients with liver damage or renal failure did not detect pharmacokinetic differences, in normal individuals
As is anyone who has been prescribed proton pump inhibitors (PPIs) or acid suppressing medications
Ideal candidates are those willing to commit to proper nutrition and regular exercise