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Facts about the skin from DermNet New Zealand Trust. Topic index: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z


Topical formulations

General principals

Topical formulations are applied directly to the skin. Advantages of this include:

Disadvantages include:

Topical formulations are made up in a vehicle, or base, which may be optimised for a particular site of the body or type of skin condition. The product may be designed to be moisturising or to maximise the penetration of an active ingredient, a medicine, into or through the skin.

The amount of the active ingredient that is absorbed through the skin depends on the following factors:

For these reasons, although generic formulations may apparently contain the same active ingredient and at the same concentration as the brand original, the effectiveness may be different.

Quantity

Normally, topical medications should be applied to the skin as a thin layer. Expect 1 gram of cream to spread out over a 10cm2 area of skin and an ointment spreads a little further. The fingertip unit (0.5g) is a guide to the amount of a cream or ointment needed to treat an area for a certain time. One fingertip unit covers one side of 2 flat hands and one gram covers both sides of the two hands.

It takes 20 to 30g of cream or ointment to cover an adult once.

Vehicles

Topical formulations contain an active ingredient, often a medication or drug or botanical, and a vehicle. The vehicle contains water, oil, alcohol or propylene glycol mixed with preservatives, emulsifiers, absorption promoters and fragrances.

The table below describes different formulations. Manufacturers interpret the definitions in various ways so a similar preparation might be called lotion, gel or cream.

Classification of topical formulations
Solution Water or alcoholic lotion containing a dissolved powder Solution formulation
Lotion Usually considered thicker than a solution and more likely to contain oil as well as water or alcohol. A shake lotion separates into parts with time so needs to be shaken into suspension before use. Lotion formulation
Cream Thicker than a lotion, maintaining its shape, e.g., 50/50 emulsion of oil and water. Requires preservative to extend shelf life. Often moisturising. Cream formulation
Ointment Semi-solid, water-free or nearly water-free (80% oil). Greasy, sticky, emollient, protective, occlusive. No need for preservative so contact allergy is rare.
May include hydrocarbon (paraffin), wool fat, beeswax, macrogols, emulsifying wax, cetrimide or vegetable oil (olive oil, arachis oil, coconut oil).
Ointment formulation
Gel Aqueous or alcoholic monophasic semisolid emulsion, often based on cellulose and liquifies upon contact with skin. Often includes preservatives and fragrances. Gel formulation
Paste Concentrated suspension of oil, water and powder Paste formulation
Aerosol foam or spray Solution with pressurised propellant Spray formulation
Powder Solid e.g. talc (a mineral) or corn starch (vegetable) Powder formulation
Solid EG antiperspirant stick. May melt on reaching body temperature e.g. suppositories. Solid formulation
Transdermal patch Drug delivery system allows precise dosing: includes an adhesive. Patch formulation

Other terms used by cosmetic and pharmaceutical manufacturers include emulsion, paint, suspension, milk, syrup, collodion, balm and mist. Formulae may have mixed ingredients with more than one type of vehicle.

Factors in the choice of vehicle or base for a topical medication include the nature of the dermatosis and its site.

When your pharmacist makes up a mixture, it is extemporaneously compounded. Galenicals is the name given to the crude ingredients (often natural in origin) – they may be added to a vehicle or to a brand-name product.

Nature of the dermatosis

Site

Special circumstances

Newborn babies

The skin barrier of full term newborn babies is nearly the same as in older children and adults. However, the barrier function in premature babies is markedly impaired.

The surface area of a baby is proportionally much greater than that of an adult. Organs such as liver, kidneys, blood and central nervous system are not fully developed. This means topically applied medications can be more likley to result in side effects and toxicity.

Pregnancy and lactation

Like oral medicines, some topical medications may be unsafe during pregnancy. These include:

Medications are classified according to their risk. The FDA classification system is often used.

Pregnancy categories used in New Zealand
Category A Drugs which have been taken by a large number of pregnant women and women of childbearing age without any proven increase in the frequency of malformations or other direct or indirect harmful effects on the fetus having been observed.
Category B1 Drugs that have been taken by only a limited number of pregnant women and women of childbearing age, without an increase in the frequency of malformation or other direct or indirect harmful effects on the human fetus having been observed. Studies in animals[1] have not shown evidence of an increased occurrence of fetal damage.
Category B2 Drugs that have been taken by only a limited number of pregnant women and women of childbearing age, without an increase in the frequency of malformation or other direct or indirect harmful effects on the human fetus having been observed. Studies in animals[1] are inadequate or may be lacking, but available data show no evidence of an increased occurrence of fetal damage.
Category B3 Drugs that have been taken by only a limited number of pregnant women and women of childbearing age, without an increase in the frequency of malformation or other direct or indirect harmful effects on the human fetus having been observed. Studies in animals[1] have shown evidence of an increased occurrence of fetal damage, the significance of which is considered uncertain in humans.
Category C Drugs that, owing to their pharmacological effects, have caused or maybe suspected of causing harmful effects on the human fetus or neonate without causing malformations. These effects may be reversible.
Category D Drugs that have caused, are suspected to have caused or may be expected to cause an increased incidence of human fetal malformations or irreversible damage. These drugs may also have adverse pharmacological effects.
Category X Drugs that have such a high risk of causing permanent damage to the fetus that they should not be used in pregnancy or when there is a possibility of pregnancy.

Tips for using topical agents

Related information

References:

On DermNet NZ:

Other websites:

Books about skin diseases:

See the DermNet NZ bookstore

Author: Dr Amanda Oakley, Dermatologist, Hamilton, New Zealand.

DermNet NZ does not provide an online consultation service.
If you have any concerns with your skin or its treatment, see a dermatologist for advice.