What Is the Cause of Enlarged Red Blood Cells?

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According to the Mayo Clinic, enlarged red blood cells, or macrocytosis, is a non-specific medical condition that has a number of different causes. Among the most common are vitamin B12 deficiency, folate deficiency, alcoholism, hypothyroidism (under-active thyroid) and liver disease. Other causes include increased red blood cell production secondary to acute blood loss or the side-effects of medications such as those used to treat cancer.