Ointments: The Practical

What is an ointment?

An ointment is a greasy semi-solid for application to the skin. It usually has no water. It is used to soften and protect the skin, to keep medicines in contact with the skin, and as a base for drugs.

Ointments:

• stay on the skin for a long time;
• do not wash off easily;
• keep water in the skin (they are occlusive), which suits dry, scaly conditions;
• are usually not suitable for weeping or hairy areas.

Ointment bases

Simple ointment is a typical base: wool fat 5%, hard paraffin 5%, cetostearyl alcohol 5% and white soft paraffin 85%. Use the formula your school gives you.

Equipment

• A balance and weights (or weighing paper)
• An ointment slab or tile (glass or porcelain) and spatulas (stainless steel)
• A dish or evaporating basin and a water bath
• A mortar and pestle
• Ointment jars or tubes, and labels

Everything must be clean and dry.

Method 1: Fusion (melting)

Used when the base is made of several ingredients, or when the drug dissolves in the melted base.

1. Weigh all the ingredients.
2. Melt the ingredient with the highest melting point first, in a dish over a water bath. Add the others in turn, in order of descending melting point.
3. Heat gently. Do not overheat, as it can spoil the base and some drugs.
4. Remove from the heat and stir until cold, so the ointment sets smooth and not grainy.
5. Add heat-sensitive or dissolved drugs when the base has cooled slightly, and stir until it is evenly mixed.
6. Pack into a jar and label.

Method 2: Incorporation (mixing into a ready base)

Used to mix a drug into a ready-made base, without heating.

1. Weigh the drug and the base.
2. If the drug is a powder, make it fine.
3. Put the powder on the tile. Add a small amount of base, or of liquid paraffin, and rub with the spatula to a smooth paste. This is levigation.
4. Add the rest of the base a little at a time, mixing thoroughly after each addition. This is called geometric dilution.
5. Mix until smooth, with no lumps or grit, and with a uniform colour.
6. Pack firmly into the jar, with no air spaces, and label.

If the drug dissolves in water, dissolve it in a very small amount of water and mix with an absorption base such as wool fat, which can take up the water.

Worked examples

Percentage w/w

Make 30 g of benzoic acid compound ointment: benzoic acid 6% w/w, salicylic acid 3% w/w, in a suitable base.

1. Benzoic acid = 30 × 6 ÷ 100 = 1.8 g
2. Salicylic acid = 30 × 3 ÷ 100 = 0.9 g
3. Base = 30 − 1.8 − 0.9 = 27.3 g

Check. 1.8 + 0.9 + 27.3 = 30 g.

Method: levigate the two powders on the tile with a little of the base, then add the rest of the base gradually, mixing until smooth.

Another example

60 g of ointment containing 15% w/w zinc oxide in simple ointment.

• Zinc oxide = 60 × 15 ÷ 100 = 9 g
• Base = 60 − 9 = 51 g

Packing, labelling and storage

• Use a wide-mouthed jar or a collapsible tube, with a tight lid.
• Pack the ointment firmly, and smooth the top.
• Label: patient's name, name and strength, directions, "For external use only", the date, and a discard date.
• Store in a cool place, because heat melts ointments.
• Advise the patient to wash the hands before and after use, and to apply a thin layer, unless told otherwise.
• A water-free ointment usually keeps longer than a cream, but the pharmacist decides the discard date.

Faults and what to do