Preservatives

Definition
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Substances used for the preservation of microbially susceptible ointment bases. Although they are not "medicinally active ingredients", they are, according to the prevailing interpretation, "effective other ingredients" of a formulation. Therefore, in contrast to excipients, which do not have a pharmacological effect of their own or do not influence it, they should not be added without the consent of the physician if possible.

Field of application/use
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Formulations which contain water in their external phase (oil-in-water lotions, oil-in-water creams, water-in-oil lotions, water-in-oil creams, see also creams, lotions) and are intended to be preserved over a longer period of time.

Notice!

Preparations with an alcohol content > 20% are not at risk. Formulations with a low water content (powders, oils or fatty ointments) are only susceptible to germ multiplication if the container is constantly opened.

Notice! Preservatives are common contact allergens (Kathon CG by 4%, benzalkonium chloride, thiomersal and parabens by 2% each), so whenever possible, one should try to do without preservatives (recipe of small amounts, storage in the refrigerator)!

Tables
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Overview of the most common substances used for the preservation of externals

Preservative	Concentration	Use
Benzoic acid	0,1-0,2%
Benzyl alcohol	1-2% and above
Benzalkonium chloride	0,002-0,02%	especially for eye, nose and ear drops
Chlorbutanol	0.3-0.5% (unstable at pH > 5)
Chlorhexidine diacetate	0,005-0,1%	especially for eye, nose and ear drops
Chlorhexidine digluconate	0,005-0,01%	especially for eye, nose and ear drops
5-chloro-2-methyl-4-isothiazolin-3-one and 2-methyl-4-iso-thiazolin-3-one, Kathon CG		mainly cosmetics
Ethanol	> 15%
Glycerol	> 30%
p-Hydroxybenzoic acid methyl ester, nipagine, solbrol	0,06%
ethyl p-hydroxybenzoate	0,07%
p-Hydroxybenzoic acid propyl ester, Nipasol, Solbrol P	0,04%
Imidazolinyl urea, Germall 115		mainly cosmetics
Potassium sorbate	0,1%
Sodium benzoate	0,1-0,2%
Phenoxyethyl alcohol	1-1,5%
phenyl mercury compounds, such as -borate, -nitrate, -acetate	0,002-0,005%	especially for nose and ear drops as well as rectally and vaginally applicable external preparations
1,2-propylene glycol	> 15%
Sorbic acid and its salts	0.1-0.15% (unstable at pH > 5)
Sorbitol	> 70%
Thiomersal	0,002-0,015%	especially for eye, nose and ear drops

Preservative	available concentration [%]	Effectiveness against Gram-positive pathogens	Effectiveness against Gram-negative pathogens	Efficacy against yeasts	Effectiveness against fungi
Chlorbutanol	0,5	very effective	very effective	weakly effective	weakly effective
Benzyl alcohol	1,0	very effective	weakly effective	weakly effective	weakly effective
Phenylethyl alcohol	1,0	very effective	very effective	weakly effective	weakly effective
Phenol, cresol	0,3	very effective	very effective	weakly effective	weakly effective
PHB methyl ester	1,18	very effective	very effective	not effective	not effective
PHB propyl ester	0,02	very effective	very effective	weakly effective	not effective
PHB mixture M + P	0,2	very effective	very effective	weakly effective	weakly effective
PHB mixture M + P + benzyl alcohol	0,2 + 0,5	very effective	very effective	very effective	very effective
Benzoic acid	0.1 (only in the pH 6 range)	not effective	not effective	very effective	very effective
Sorbic acid	0.2 (only in the pH 6 range)	not effective	not effective	very effective	very effective
Chlorhexidine	0,005-0,1	very effective	very effective	weakly effective	weakly effective
Benzalkonium chloride	0,01	very effective	weakly effective	very effective	very effective
Phenylmercurinittate	0,001	very effective	very effective	very effective	very effective
Thiomersal	0,02	very effective	very effective	very effective	very effective

Preservative	Optimal pH range
	2	3	4	5	6	7	8	9	10
Chlorbutanol	very effective	very effective	very effective	very effective	not effective	not effective	not effective	not effective	not effective
Benzyl alcohol	very effective	very effective	very effective	very effective	weakly effective	weakly effective	not effective	not effective	not effective
Phenol, cresol, p-chloro-m-cresol	very effective	very effective	very effective	weakly effective	weakly effective	weakly effective	weakly effective	weakly effective	weakly effective
PHB methyl ester	very effective	very effective	very effective	very effective	very effective	weakly effective	weakly effective	weakly effective	not effective
PHB propyl ester	very effective	very effective	very effective	very effective	very effective	weakly effective	weakly effective	weakly effective	not effective
Sodium sulphite	not effective	not effective	not effective	not effective	not effective	very effective	very effective	very effective	not effective
Benzoic acid (pKa value 4.194)	very effective	very effective	very effective	very effective	very effective	not effective	not effective	not effective	not effective
Sorbic acid (pKa value 4,76)	very effective	very effective	very effective	very effective	very effective	not effective	not effective	not effective	not effective
Chlorhexidine	not effective	not effective	not effective	very effective	very effective	very effective	very effective	not effective	not effective
Benzalkonium chloride	not effective	very effective	very effective	very effective	very effective	very effective	very effective	weakly effective	weakly effective
Phenylmercurinitrate	not effective	not effective	not effective	not effective	not effective	very effective	very effective	very effective	very effective
Thiomersal	very effective	very effective	very effective	very effective	very effective	very effective	not effective	not effective	not effective
Phenylethyl alcohol	very effective	very effective	very effective	very effective	very effective	very effective	not effective	not effective	not effective