Symptoms can range widely because vitamin B12 is required by all cells of the body. Some of the common symptoms of vitamin B12 deficiency are weakness, apathy, memory loss, acidity, loss of weight, nausea and vomiting, anaemia, mental confusion, delusions, paranoia, respiratory symptoms, hives and other symptoms of allergy. A long-term deficiency can lead to heart attacks or stroke – the very problems that a healthy vegan diet can prevent. Long-term vitamin B12 deficiency can cause irreversible damage to the brain and nervous system.