Purified water is the first choice as a basis in mouthwashes and gargles. Aqueous solutions of glycerine or sorbitol are good alternatives. The use of high concentrations of glycerine in mouthwashes is controversial. By dehydrating the mucosa these solutions would have the opposite effect to that which they are meant to have [13].

Gingival solutions with antiseptics or local anaesthetics may also be formulated in mixtures of water and glycerine. In this case glycerine serves the purpose of raising the viscosity of the vehicle and improving adherence to the mucosa. This is relevant because gingival solutions are used on local ailments in the oral cavity.

When saliva production is insufficient, many different liquids can be used. They vary from glycerol with citric acid to the saliva substitutes already mentioned in Sect. 7.1. Citric acid stimulates the saliva production in the salivary glands, and therefore raises the amount of saliva in the mouth. If there is damage to the oral mucosa, it is best to avoid ethanol and propylene glycol, because these solvents may cause irritation, A German hospital developed an alcohol free formulation with benzydamine, especially for patients on chemotherapy or radiation treatment [14]. Benzydamine is a NSAID with local anaesthetic and antiseptic properties.

It is clear that surface tension and the viscosity of the vehicle will influence the effect of a mouthwash, but there is still little investigated in this area.

For reasons of taste mouth preparations should be slightly acid or neutral. Instability or ineffectiveness of the active ingredient can make deviations in pH necessary. In Lidocaine hydrochloride oral gel 20 mg/ml FNA the pH is adjusted to 6.8, because at that pH part of the lidocaine will be in the base form (Table 7.2). Therefore, at that pH the numbing effect will be much better than in an acid solution.

More information about the influence of pH on chemical and physical stability of solutions and suspensions can be found in Sect. 22.​2.

Mouthwashes need not be made iso-osmotic. But strongly hyperosmotic solutions may hurt in case of lesions in the mouth. Sorbitol solution, because of its viscosity sometimes used as vehicle for suspensions, is strongly hyperosmotic. For patients with lesions a formulation sometimes has to be adapted. For instance many chlorhexidine mouthwashes contain sorbitol as a flavouring and ethanol as preservative. In the following adapted version both are omitted (Table 7.3).

In the preparation of viscous mouthwashes common viscosity enhancers are used. Mostly cellulose derivatives are used, such as hypromellose, but sometimes tragacanth is preferred. Nowadays tragacanth with a good microbiological quality can be purchased. The main advantage of tragacanth gels is their resemblance to oral mucus, which makes flavouring easier. An example is Tetracycline mouthwash 5 % FNA (Table 7.4).

As most preparations for the oropharynx are aqueous solutions, growth of micro-organisms is possible. Therefore, the addition of a preservative is generally needed, except for those preparations that have intrinsic preservative properties. In this category there are many preparations with glycerol, propylene glycol or ethanol, provided that these are present in a sufficiently high concentration. See also Sect. 23.​8. An example is glycerol with citric acid, see Table 7.5

This solution contains so much glycerol that addition of a preservative is not necessary. Zinc chloride and alumen gargle FNA does not need preservation due to the low pH (2.3–2.7) and the presence of salicylic acid Table 7.6.

If a preservative is needed in a preparation for the oropharynx the following considerations are important: