SKU Vitamin B12 (5000 mcg/ml) [100 mL] Cyanocobalamin 5000 mcg is for use in managing vitamin B12 deficiencies associated with cobalt deficiency, inadequate intake, or intestinal malabsorption in cattle, horses, sheep, and swine
This may be if the tophus is on the bottom of the foot, and the person has difficulty walking on it, or on the side of the foot making it hard to wear shoes
Using expired b12 could certainly result in you not receiving the proper dosage due to deterioration as you now likely suspect given your setback
The concern is dosing accuracy, not safety
The medication dramatically lowers the barrier to achieving these goals by reducing hunger and cravings but it works in partnership with your lifestyle choices, not in place of them