Antioxidants are included in pharmaceutical solutions or suspensions to enhance the stability of therapeutic agents that are susceptible to chemical degradation by oxidation. Typically antioxidants are molecules that are redox systems that exhibit higher oxidative potential than the therapeutic agent or, alternatively, are compounds that inhibit free radical-induced drug decomposition.

Typically in aqueous solution antioxidants are oxidised (and hence degraded) in preference to the therapeutic agent, thereby protecting the drug from decomposition. Both water-soluble and
water-insoluble antioxidants are commercially available, the choice of these being performed according to the nature of the formulation.

Suitable antioxidants used are as follows.
  • Ascorbic acid derivatives such as ascorbic acid, erythorbic acid, Na ascorbate.
  • Thiol derivatives such as thioglycerol, cysteine, acetylcysteine, cystine, dithioerythreitol, dithiothreitol, glutathione
  • Tocopherols
  • Butylated hydroxyanisole (BHA)
  • Butylated hydroxytoluene (BHT)
  • Sulfurous acid salts such as sodium sulfate, sodium bisulfite, acetone sodium bisulfite, sodium
  • metabisulfite, sodium sulfite, sodium formaldehyde sulfoxylate, and sodium thiosulfate.
  • Nordihydroguaiaretic acid

Antioxidants used for aqueous formulations include: sodium sulphite, sodium metabisulphite, sodium formaldehyde sulphoxylate and ascorbic acid.

Antioxidants used in oil-based solutions include: butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA) and propyl gallate.

Typically antioxidants are employed in low concentrations (0.2% w/w) and it is
usual for the concentration of antioxidant in the finished product to be markedly less than the initial concentration, due to oxidative degradation during manufacture of the dosage form.

Antioxidants may also be employed in conjunction with chelating agents, e.g. ethylenediamine tetraacetic acid, citric acid, that act to form complexes with heavy-metal ions, ions that
are normally involved in oxidative degradation of therapeutic agents.