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https://www.mayoclinic.org/drugs-supplements-vitam…
Vitamin B-12 - Mayo Clinic
People with digestive conditions such as celiac disease and Crohn's disease also are at risk of low vitamin B-12 levels. People with very low vitamin B-12 levels can have symptoms such as fatigue, muscle weakness, stomach problems, nerve damage, vision problems and mood changes. They also may have low iron, called anemia, or low blood counts.