Can lack of vitamin b12 cause depression?

Yes, but: It is rare that B12 deficiency will cause only depression, and nothing else. There is a long list of symptoms that go with B12 deficiency (anemia, fatigue, bleeding gum, for example). But, it is certainly possible. So the best thing to do is to get blood level of B12 checked. Read more here: http://www.Nlm.Nih.Gov/medlineplus/ency/article/000574.Htm.