The vitamers of a particular vitamin are all of the chemical compounds which exhibit vitamin activity. Very commonly "vitamins" are not single compounds, but rather each vitamin, which is defined by its biological activity, not its structure, is actually represented by a number of substances, all of which show vitamin activity. Typically, the vitamin activity of multiple vitamers is due to the body's limited ability to convert one vitamer to another, or many vitamers to the same enzymatic cofactor(s). This is the case even though (as part of the definition of vitamin) the body cannot completely synthesize an optimal amount of vitamin activity from simple foodstuffs, without a minimal vitamer molecule as a basis.
Typically not all vitamers possess exactly the same vitamin potency, per mass. This is due to differences in absorption and interconversion of the various different vitamers of a vitamin. Often for the same reason, the toxicity of vitamers varies by molecule, as is the case with vitamin E.
A set of chemicals may be (but is not always) grouped under an alphabetized vitamin "generic descriptor" title, such as "vitamin A," which (for example) includes retinal, retinol, and many carotenoids. Examples of vitamers include cyanocobalamin, hydroxocobalamin, methylcobalamin, and 5-deoxyadenosylcobalamin (adenosylcobalamin—AdoB-12), which are all vitamers of B-12, and thus all possess "B-12 activity". Another example is that both niacinamide and nicotinic acid (niacin) have vitamin B-3 activity.