Vitamin B12 is a water-soluble vitamin that contains the mineral cobalt and is therefore also referred to as cobalamin.
Vitamin B12 is readily available in foods of animal origin but not available in plant-based foods. However, fortified breakfast cereals and fortified nutritional yeasts are good sources of vitamin B12 that have high bioavailability. 
Also, vitamin B12 has the largest and most complex chemical structure of all the vitamins.
Toxicity is rare for this nutrient as it’s a water-soluble vitamin and the body removes excess amount instantly. [1,2,4]