What can cause someone to be lacking in these important B vitamins?


B6 deficiency can occur if taking anticonvulsants, isoniazid, hydralazine, corticosteroids, and penicillamine; B9 deficiency can occur if taking phenytoin, trimethoprim-sulfamethoxazole, methotrexate, and sulfasalazine; B12 deficiency can occur if taking metformin, proton pump inhibitors (PPIs), antacids, long-term antibiotics, and antidepressants.

Alcohol causes you to increase kidney filtration making it more likely for your kidneys to rid your body of B vitamins more quickly. 
Vegetarian or Vegan Diets
B12 is mainly found in animal products, but some dairy and cereal products are fortified with B12, making individuals who do not consume animal products for personal or religious reasons at a higher likelihood of being deficient in B12. 
Gut Conditions
Since all B vitamins must be absorbed by the gut, individuals with certain gut conditions like Celiac, Crohn’s, ulcerative colitis, or individuals who have had bariatric surgery are more likely to be deficient.