Preparation of Pharmaceutical Ointments
Ointments are semi-solid preparations for application to the skin or mucous membranes. Their basis is almost always anhydrous and generally contains one or more medicaments. They usually contain a medicament dissolved, suspended or emulsified in the base. The main rule of the local therapy is that if the skin is dry, use an ointment.
Ointments are used for their emollient effect, protective action to the skin and for topical medication. E.g.:
• Antibiotics –bacitracin, neomycin, mupirocin, etc.
• Antifungal agents –compound benzoic acid ointment
• Anti-inflammatory agent- betamethasone valerate, hydrocortisone, etc.
The base of a traditional ointment consists of a mixture of waxes, fats and oils:
• Waxes – solid and hard at room temperature
• Fats – semi-solid, soft at room temperature
• Oils – liquid at room temperature.
A change in temperature can affect the physical state of a base (e.g. coconut oil is solid in winter but is more likely to be liquid in summer). The addition of a wax to an ointment makes the preparation smoother and lighter in consistency. Altering the proportions of oil, fat and wax in the ointment may vary the consistency. For example, extra wax will make the ointment stiffer; extra oil will make the ointment less viscous. The proportions used may vary depending on storage or the climatic conditions (e.g. whether the product is intended for use in the Tropics or in the Arctic).
General method for incorporating powders into an ointment base
Soluble solids. Soluble solids should be added to the molten fatty bases at the lowest possible temperature and the mixture stirred until cold. Alternatively, if using a pre-prepared base, soluble solids may be incorporated using the method employed for insoluble solids.
Insoluble solids. Insoluble solids should be incorporated using an ointment tile and spatula. If there is more than one powder to be added these should be mixed in a mortar using the ‘doubling-up’ method.
General method for incorporating liquids into an ointment base
Non-volatile, miscible liquids
Non-volatile, miscible liquids may be mixed with the molten fat in the evaporating basin. Alternatively, if a pre-prepared base is used, then incorporate as for volatile or immiscible liquids.
Volatile or immiscible liquids
Volatile or immiscible liquids (e.g. coal tar solutions) should be triturated with the ointment on the ointment tile. A very small amount of the ointment should be placed on the tile and a ‘well’ made in the centre. Traditionally, small quantities of liquid should be gently folded in to avoid splashing. An alternative method is to spread a small amount of the ointment on the tile and then ‘score’ it with a spatula. Then add small quantities of the liquid and fold into the base gently.
If using coal tar or other volatile ingredients, these should not be weighed until immediately before use and the beaker in which it has been weighed should be covered with a watch glass to prevent evaporation.
A well-made ointment is
(a) Uniform throughout i.e. it contains no lumps of separated high melting point ingredients of the base, there is no tendency for liquid constituents to separate and insoluble powders are evenly dispersed.
(b) Free from grittiness, i.e. insoluble powders are finely subdivided and large lumps of particles are absent. Methods of preparation must satisfy this criteria. Two mixing techniques are frequently used in making ointments:
1. Fusion, in which ingredients are melted together and stirred to ensure homogeneity.
2. Trituration, in which finely-subdivided insoluble medicaments are evenly distributed by grinding with a small amount of the base or one of its ingredients followed by dilution with gradually increasing amounts of the base.
1. Ointments prepared by Fusion method
This involves melting together the bases over a water bath before incorporating any other in gredients. The ointment base may include a mixture of waxes, fats and oils, of which some are solid at room temperature and others are liquid:
• Hard – Paraffin BP, Beeswax BP, Cetostearyl Alcohol BP
• Soft – Yellow and White Soft Paraffin BP, Wool Fat BP
• Liquid – Liquid Paraffin BP and vegetable oils. Method (fusion)
1. Always make excess as transference losses will always occur.
2. Determine the melting points of the fatty bases and then melt together. Starting with the base with the highest melting point, each base should be melted at the lowest possible temperature as the mixture progressively cools.
3. Add the ingredients to an evaporating basin over a water bath to avoid overheating – use a thermometer to check the temperature regularly.
4. As the first base cools add the ingredients with decreasing melting points at the respective temperatures, stirring continuously to ensure a homogeneous mix before leaving to set. It is important to stir gently to avoid incorporating excess air, which could result in localised cooling and a lumpy product.
2. Ointment prepared by trituration
This method is applicable in the base or a liquid present in small amount.
1. Solids are finely powdered are passed through a sieve (# 250, # 180, #125).
2. The powder is taken on an ointment-slab and triturated with a small amount of the base. A steel spatula with long, broad blade is used. To this additional quantities of the base are incorporated and triturated until the medicament is mixed with the base.
3. Finally liquid ingredients are incorporated. To avoid loss from splashing, a small volume of liquid is poured into a depression in the ointment and thoroughly incorporated before more is added in the same way. Splashing is more easily controlled in a mortar than on a tile.