Is methadone a cause of pernicious anemia?

No. Unaware of any association. True pernicious anemia is an autoimmune disease, caused by antibodies to the cells in our stomachs that produce intrinsic factor, which is needed to absorb vitamin B12 from our intestines. It may be hereditary or acquired. There are other causes of B12 deficiency, including stomach and intestinal diseases/surgery, and lack of B12 in diet. A hematologist can help.
No. I wish every question were this easy to answer. Methadone treatment is a great blessing for many people. Classic pernicious anemia is an autoimmune disease involving the stomach interfering with B12 absorption. It is possible for a person taking Methadone to develop it, but it would be a coincidence.