antioxidant

An antioxidant is an excipient that takes a chemical bullet for the drug — it is preferentially oxidised, or it interrupts the oxidation chain, so the active ingredient survives. It is the formulation equivalent of squeezing lemon juice on sliced apple to stop it browning.

Antioxidants work by different mechanisms. True (chain-breaking) antioxidants such as butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT) and alpha-tocopherol donate a hydrogen atom to quench free radicals and halt the autoxidation chain; they suit oil-based and lipophilic systems. Reducing agents (oxygen scavengers) such as sodium metabisulfite, sodium sulfite and ascorbic acid have a lower oxidation potential than the drug and so are consumed in its place; they suit aqueous systems. A third group, synergists like chelating agents, are not antioxidants themselves but boost the others by removing catalytic metals.

Because they are sacrificial, antioxidants are used up over the product's life, which is itself a stability consideration: enough must be present to last the shelf life. They are chosen for solubility in the relevant phase, for compatibility, and at the lowest effective concentration.

A genuine caveat: some antioxidants carry their own risks. Sulfites can provoke hypersensitivity reactions in susceptible patients (notably some asthmatics), and certain antioxidants can themselves discolour or react with the drug — so their use is a balance, not a free win.