Preservatives and Antioxidants Database

Last Review: October 2005

Preservatives Antioxidants Adjuvants/Synergists

Preservation is the prevention or inhibition of microbial growth. In pharmacy, this is commonly accomplished by the addition of a preservative to a product, with the primary purpose of minimizing microbial growth (as in oral liquids, topicals, etc.), or for preventing microbial growth (as in sterile preparations such as parenterals).

Several factors are involved in the selection of a preservative, including concentration, pH, taste, odor and solubility. Some preparations, such as syrups, are inherently preserved by the high concentration of sugar present, which acts as an osmotic preservative.

In most preparations, however, a suitable preservative must be selected and attention paid to assuring preparation of a stable product. A preservative must be nontoxic, stable, compatible and inexpensive and have an acceptable taste, odor and color. It should also be effective against a wide variety of bacteria, fungi and yeasts.

Autoxidation is the reaction of any substance with molecular oxygen. These reactions can be catalyzed by heavy metals, especially those with two or more valency states (cobalt, copper, iron, nickel). Other catalysts include hydrogen and hydroxyl ions, since the redox potential of many compounds is pH related. These catalysts can reduce the onset time and increase the rate of autoxidation reactions. The catalysts, in essence, increase the rate of formation of free radicals.

Antioxidants are among a number of adjuvants commonly added to pharmaceutical systems to enhance physical and chemical stability. Antioxidants are added to minimize or retard oxidative processes that occur with some drugs or excipients upon exposure to oxygen or in the presence of free radicals. These processes can often be catalyzed by light, temperature, hydrogen ion concentration, presence of trace metals, or peroxides.

Some substances prone to oxidation include unsaturated oils/fats, compounds with aldehyde or phenolic groups, colors, flavors, sweeteners, plastics and rubbers, the latter being used in containers for products.

Oxidation may manifest as products with an unpleasant odor, taste, appearance, precipitation, discoloration or even a slight loss of activity. The term rancidity refers to many typical off-flavors that result from autoxidation of unsaturated fatty acids that are present in oils and fats, and it affects many oils and fats. The distinct rancid odor may result from short-chain, volatile monomers resulting from the cleavage of the longerchain, less volatile oils and fats.


Select an agent by clicking on the name
Benzalkonium chloride Benzethonium chloride Benzoic Acid and salts
Benzyl alcohol Boric Acid and salts Cetylpyridinium chloride
Cetyltrimethyl ammonium bromide Chlorobutanol Chlorocresol
Chorhexidine gluconate or Chlorhexidine acetate Cresol Ethanol
Imidazolidinyl urea Metacresol Methylparaben
Nitromersol o-Phenyl phenol Parabens
Phenol Phenylmercuric acetate/nitrate Propylparaben
Sodium benzoate Sorbic acids and salts ß-Phenylethyl alcohol


Select an agent by clicking on the name
a-tocopherol acetate Acetone sodium bisulfite Acetylcysteine
Ascorbic acid Ascorbyl palmitate Butylated hydroxyanisole (BHA)
Butylated hydroxytoluene (BHT) Cysteine Cysteine hydrochloride
d- a-tocopherol natural d- a-tocopherol synthetic Dithiothreitol
Monothioglycerol Nordihydroguaiaretic acid Propyl gallate
Sodium bisulfite Sodium formaldehyde sulfoxylate Sodium metabisulfite
Sodium sulfite Sodium thiosulfate Thiourea


Select an agent by clicking on the name
Citric acid EDTA (ethylenediaminetetraacetate) and salts Hydroxyquinoline sulfate
Phosphoric acid Tartaric acid