How to prepare Ointments?

Ointments can be prepared either by mechanical incorporation or by fusion methods. Irrespective of the method employed for the preparation, ointments should be smooth and free from granular or gritty particles. In compounding of ointments, the following general considerations are observed.

(i) If insoluble substances are to be incorporated in the ointment base then they should be in impalpable powder form.

(ii) For efficient incorporation of insoluble substances they should first be levigated with a little quantity of base to form a smooth cream and then incorporated into the remainder of the base.

(iii) Water-soluble salts are best incorporated by dissolving them in a small quantity of water and then incorporating in the base.

(iv) Drugs soluble in ointment bases may also be incorporated by fusion (melting the highest melting point ingredient of the base and mixing the medicament into it). Remaining ingredients are then added and mixed by stirring.

Preparation of Ointments by Mechanical Incorporation

This can be achieved by the use of (i) mortar and pestle, (ii) ointment slab and spatula, and (iii). an ointment mill. Mechanical method of incorporation is particularly advantageous when the substance to be incorporated into ointment base must be in a fine state of subdivision.

(a) Mortar and pestle:

This method is used to a limited extent in compounding practice particularly when large quantities of liquids are to be incorporated in a base or when exceptionally large quantities of the ointments are to be prepared. Compounding a homogeneous ointment in a mortar and pestle is not as simple as compounding of a powder.

(b) Ointment slab:

In this case both mixing and size reduction of insoluble medicaments are better than the previous method. The powder is first rubbed with a small quantity of the base to form a concentrated ointment base containing a finely divided powder uniformly distributed in it.

The concentrated ointment is then gradually diluted with remaining quantity of the base by rubbing with a spatula. A small quantity (approximately 5%) of oil or oil-soluble substances can be used as a levigating agent. Large amounts of levigating agents may cause undue softening of the finished preparation.

The spatula should be of stainless steel with a long, broad flexible blade. When steel spatula cannot be used for reasons of reacting with certain drugs like iodine, salicylic acid, and mercury salts etc., a hard rubber spatula or a wooden tongue depressor may be used.

(c) Ointment mill:

Ointment mills are particularly suitable for large scale manufacture of ointments although small mills are available for laboratory scale ointment preparation. Ointments containing gritty particles are also passed through the ointment mill to ensure further uniformity and smoothness.

The problems of stability of ointments are mostly concerned with the initial and ‘in-use’ microbial contamination and its increase. Many ophthalmic ointments are found to be heavily contaminated with pathogenic micro-organisms. The main source of microbial contamination in ointments is water which supports the growth of micro-organisms.

To safeguard the stability of the packaged product the following points are borne in mind.

(i) In general, the pharmacist or the manufacturer should follow Good Manufacturing Practice (GMP).

(ii) Use of preservatives to protect against the contamination, deterioration or spoilage of ointment bases by bacteria and fungi.

(iii) Use of antioxidants when there is possibility of oxidative degradation of the base.

(iv) Use of chelating agents where the presence of traces of metallic ions is anticipated. The traces of metallic ions, if present, may catalyse oxidative degradation.

(v) If the metallic ions are chelated their catalytic effect is nullified.

(vi) The immediate container should not permit evaporation of water from the packaged ointments otherwise an emulsion base may lose aqueous phase and others may become dry and hard.

(vii) The containers should ensure not only the sterility of ophthalmic ointments initially but also up to a time till whole of the preparation is consumed.

(viii) Due to wide variations in the climatic conditions in different regions in tropical countries, maintenance of desirable consistency of an ointment is also an important consideration.

(ix) Most of the problems related to the stability of packaged ointments can be solved by dispensing them as single application capsules although it works out to be more expensive. This is particularly recommended in case of ophthalmic ointments where sterility and absence of particles are the prime considerations.