The purpose of this study is to evaluate the effects of gastric bypass surgery and sleeve gastrectomy on vitamin B12 values in dogs. Gastric dilation and volvulus (GDV) is an acute disease with a high mortality rate in dogs. Either gastric bypass surgery or sleeve gastrectomy can be utilized as surgical treatments for GDV. These techniques can provide increased success and fewer complications, however, one undesirable outcome can be vitamin B12 insufficiency. Three groups of dogs received gastric surgeries, one group underwent gastric rotation and derotation with no tissue gastric incisions, the two other groups had gastric bypass surgery and sleeve gastrectomy. Vitamin B12 level was evaluated two weeks prior to surgery, at the time of surgery, two, four, and six weeks after surgery. The results demonstrated no inter-group differences two weeks prior to surgery until four weeks after surgery. However, six weeks post-operation, significant differences between the sleeve gastrectomy group, and the other two groups were shown in vitamin B12 levels. Gastric bypass surgery and sleeve gastrectomy can both be utilized as new surgical managements of GDV. However, as we have demonstrated in this article, an important expected side effect is vitamin B12 insufficiency, in dogs that receive sleeve gastrectomy for GDV treatment. This necessary element must be substituted artificially for all dogs which had received these surgical managements.