The naturally occurring suspending agents such as tragacanth, acacia, xanthan gum are susceptible to microbial contamination. If suspension is not preserved properly then the increase in microbial activity may cause stability problem such as loss in suspending activity of suspending agents, loss of color, flavor and odor, change in elegance etc. Antimicrobial activity is potentiated at lower pH.

Preservatives are included in pharmaceutical dosage form to control the microbial bioburden of the formulation. Ideally, preservatives should exhibit the following properties:
  • possess a broad spectrum of antimicrobial activity encompassing Gram-positive and Gram-negative bacteria and fungi
  • be chemically and physically stable over the shelf-life of the product
  • have low toxicity.
  • should not be Adsorbed on to the container
  • should be compatible with other formulation additives.
  • Its efficacy should not be decreased by pH.
This occurs most is commonly in antacid suspensions because the pH of antacid suspension is 6-7 at which parabens, benzoates and sorbates are less active. Parabens are unstable at high pH value so parabens are used effectively when pH is below 8.2. Most commonly observed
incompatibility of PABA (Para amino benzoic acid) esters is with non-ionic surfactant, such as polysorbate 80, where PABA is adsorbed into the micelles of surfactant.

The combination of two or more preservative has many advantages in pharmaceutical system such as
  • Wide spectrum of activity
  • Less toxicity
  • Less incidence of resistance
  • Preservatives can be used in low concentration.
Propylene glycol is added to emulsions containing parabens to reduce loss to micelles. 

List of Preservatives and Their Optimal Concentration.

Name of preservatives Concentration range
Propyleneglycol 5-10 %
Disodium edentate 0.1 %
Benzalkonium chloride 0.01-0.02 %
Benzoic acid 0.1 %
Butylparaben 0.006-0.05 % oral suspension
0.02-0.4 % topical formulation
Cetrimide 0.005 %
Chlorobutanol 0.5 %
Phenyl mercuric acetate 0.001-0.002 %
Potassium sorbate 0.1-0.2 %
Sodium benzoate 0.02-0.5 %
Sorbic acid 0.05-0.2 %
Methyl paraben 0.015-0.2 %

alkyl esters of parahydroxybenzoic acid (0.001–0.2%). Usually a combination of two members of this series is employed in pharmaceutical solutions, typically methyl and propyl parahydroxybenzoates (in a ratio of 9:1). The combination of these two preservatives enhances the antimicrobial spectrum. Now a days, combination of phenylethyl alcohol, phenoxetol and benzalkonium chloride are used in eye drops. EDTA (ethylenediaminetetra-acetate) is also used in combination with other preservative.

Factors affecting preservative efficacy in oral suspension
The activity of a preservative is dependent on the correct form of the preservative being available in the formulation at the required concentration to inhibit microbial growth (termed the minimum inhibitory concentration: MIC). Unfortunately, in many liquid formulations, the concentration of preservative within the formulation may be affected by the presence of other excipients and by formulation pH. Factors that directly affect the efficacy of preservatives in oral solutions include:
  1. the pH of the formulation
  2. the presence of micelles
  3. the presence of hydrophilic polymers.
The pH of the formulation. In some aqueous formulations the use of acidic preservatives, e.g. benzoic acid, sorbic acid, may be problematic. 

Active form of preservative may be ionized or unionized form. For example active form of benzoic acid is undissociated form. The pKa of benzoic acid is 4.2. Benzoic acid is active below pH 4.2 where it remains in unionized form. The activity of the unionised form of the acid in this respect is due to the ability of this form to diffuse across the outer membrane of the microorganism and eventually into the cytoplasm. The neutral conditions within the cytoplasm enable the preservative to dissociate, leading to acidification of the cytoplasm and inhibition of growth.

The presence of micelles
If the preservative exhibits lipophilic properties (e.g. the unionised form of acidic preservatives, phenolics, parabens), then partition of these species into the micelle may occur, thereby decreasing the available (effective) concentration of preservative in solution. 

The presence of hydrophilic polymers
It has been shown that the free concentration of preservative in oral solution formulations is reduced in the presence of hydrophilic polymers, e.g. polyvinylpyrrolidone, methylcellulose. This is due to the ability of the preservative to interact chemically with the dissolved polymer. As described above, this problem is addressed by increasing the concentration of preservative in the formulation. In certain circumstances the preservative may be incompatible with hydrophilic polymers in the formulation due to an electrostatic interaction. Therefore, cationic hydrophilic polymers should not be used in conjunction with acidic preservatives in oral solution formulations. 

Preservative efficacy is expected to be maintained in glass container if the closure is airtight, but now a days plastic container are widely used where great care is taken in selection of preservative. The common problem associated with plastic container is permeation of preservatives through container or adsorption of preservatives to the internal plastic surface. The use of cationic antimicrobial agents is limited because as they contain positive charge they alter surface charge of drug particles. Secondly they are incompatible with many adjuvants. 

Most common incidents, which cause loss in preservative action, are,
  • Solubility in oil
  • Interaction with emulsifying agents, suspending agents
  • Interaction with container
  • Volatility