Antioxidants are naturally found in beer and may have a positive effect on health. An antioxidant is a substance that protects against the oxidation of other molecules. Oxidation is defined as the loss of electrons. Antioxidants function by interfering with the siphoning of electrons from molecules. They may do this by themselves being preferentially oxidized, thereby preventing other materials being oxidized; by blocking the action of oxidizing systems; or by donating electrons. Antioxidants may be endogenous (i.e., native to the raw materials of brewing), exogenous (i.e., added), or both.

Examples of endogenous antioxidants are polyphenols (such as catechin), phenolic acids (e.g., ferulic acid), and Maillard reaction products and enzymes (notably superoxide dismutase, catalase, and peroxidases). See maillard reaction, phenolic, and polyphenols. Exogenous antioxidants would include ascorbic acid, which, although present (as vitamin C) in many living systems, is not substantially present in those organisms significant in brewing. Sulfur dioxide (metabisulfite) is both endogenous (it is a product of yeast metabolism) and exogenous.

Antioxidants in a brewing context have two significant roles. First, they protect against the oxidation of wort, yeast, and beer, thereby extending beer shelf life and yeast viability. Second, their presence in beer is relevant to the impact of beer on the health of the drinker. The antioxidants in beer are derived from both the malt and the hops, but the levels found will depend on the style of beer and the raw materials and brewing processes used. Beer contains more than twice as many antioxidants as white wine (of equivalent alcohol content) but only half the amount in red wine, although the antioxidants in beer tend to be smaller molecules than in wine and may be more readily absorbed by the body. Researchers working on animals have suggested a direct effect of antioxidants in beer to reduce the risk of cardiovascular disease.