Vitamin B12 (cobalamin), nature’s most chemically complex vitamin, also has been called its most beautiful, owing to the rich, dark red color of its crystals. But praise for the last B vitamin to be identified is hardly limited to its physical properties: B12’s vital role in the treatment of pernicious anemia has won it a dedicated following among physicians for decades. Vitamin B12 also has been shown to guard against stroke and heart disease, and contribute to relieving asthma, bursitis,...