Being me (sciencey minded) I've done my own trials on myself with in-date and out-of-date ampoules and found no difference
Without adequate intake of B12, vegans can easily develop deficiency over time, leading to symptoms such as fatigue, weakness, constipation, loss of appetite, weight loss, and nerve problems therefore the researchers recommend that vegans and vegetarians should add vitamin B12 to their diet by eating fermented foods, particular types of mushroom that contain the vitamin and B12-enriched vegetables
Health Canada said they may even contain contaminants from heavy metals, fibers, glass and plastic to bacteria, fungi and endotoxins
But these arent all of the medical reasons and there are also medications that interfere with B12 absorption
After reconstitution: Keep refrigerated