Vitamin B12 and folate perform several important functions in the body, including keeping the nervous system healthy. A deficiency in either of these vitamins can cause a wide range of problems, including: extreme tiredness a lack of energy pins and needles (paraesthesia) a sore and red tongue mouth ulcers muscle weakness disturbed vision psychological problems, which may include depression and confusion problems with memory, understanding and judgement Some of these problems can also occur if you have a deficiency in vitamin B12 or folate, but don't have anaemia.