topical

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In medicine, a topical medication is applied to body surfaces such as the skin or mucous membranes, for example the vagina, anus, throat, eyes and ears.

Some hydrophobic chemicals such as steroid hormones can be absorbed into the body after being applied to the skin in the form of a cream, gel or lotion. Transdermal patches have become a popular means of administering some drugs for birth control, hormone replacement therapy, and prevention of motion sickness. Chloramphenicol is an example of an antibiotic that may be used topically.

In dentistry, a topical medication may also mean one that is applied to the surface of teeth.

"Topical" is derived from the Ancient Greek topos (plural: topoi), "place" or "location".

Classes of topical medications

In reality, what a manufacturer chose to list on the label of a topical medication might be completely different than what the medication is. Example is Eucerin cream, it is more of an ointment than a cream. A medication's potency often is changed with its base. Example is that some topical steroid will be classified one or two strengths higher when moving from cream to ointment. The theory is that an oitment base is more occlusive and will drive the medication into the skin more rapidly than a solution or cream base.^[1]

The manufacturer of each topical products has total control over the content of the base of a medication. One generic cream might be more acidic than another, and can cause skin irritation. Example: a vaginal formulation of miconizole cream might irritate the skin less than an athlete foot formulation of miconizole cream. Name brand topical medication might result in completely different clinical outcome than a generic, even though the active ingredient is the same. No comparative potency labeling exists to ensure equal efficacy between generic and brand name topical steroids (percentage of oil vs water dramatically affect the potency of topical steroid). Studies have conformed that the potency of generic topical steroid products vs. name brand products (however, ointment base might be more similar). Example is the case of brand name Valisone cream and Kenalog cream in clinical studies have demonstrated significantly better vasoconstrictions than some generics.^[2] But, again, in a simple base like an ointment, this might not be the case. The key point here is, generic topical might not be equivalent to name brand topical; mainly due to the variable composition of the base.

In dermatology, the base of a topical medication is often as important as the medication itself. It is extremely important to receive a medication in the correct base, before applying to the skin. A pharmacist should not substitute an ointment for a cream, or vice-versa; as the potency of the medication can change. Some physicians use an ointment to replace the waterproof barrier of the inflammed skin in the treatment of eczema, and a cream might not accomplish the same clinical intention.

The following guideline tends to be the compounding formula for many topical steroid. However, user will need to refer to the ingredients - as compounding pharmacies are not obligated to follow any set of rules on ingredient of base mix.

Solution

Tends to be of low viscosity. More of a liquid. Often compounded with water or alcohol. Frequently, in topical steroid, alcohol predominate. Tends to cause drying.

Lotion

Thicker and tends to be more emollient in nature than solution. Tends to be oil in water. Can be drying if contains excessive alcohol.

Shake Lotion

A mixture that separates into two or three parts with time. Frequently an oil mixed with a water-based solution. Needs to be shaken into suspension before use.

Cream

Thicker than lotion, and maintain it shapes when removed from its container. Tends to be moderate in moisturizing tendency. Tends to be oil in water emulsion for topical steroid products.

Ointment

Usually refer to a greasy thick oil with high viscosity. Usually very moisturizing, and good for dry skin. Often water in oil, or pure oil base. Frequently compounded with petroleum jelly, lanolin, or coco butter.

Gel

Thicker than a solution. Some will melt at body temperature. Tends to be cellulose cut with alcohol or acetone. Tends to be drying.

Foam

Can be seen with topical steroid marketed for the scalp.

Transdermal patch

Can be a very precise time release method of delivering a drug. Cutting a patch in half might affect the dose delivered. The patch system relies on steady diffusion through semipermeable membrane. Cutting a patch might cause rapid dehydration of the base of the medicine, and affect the rate of diffusion.

References

• Goodman & Gilman's The Pharmacological Basis of Therapeutics
• Dr. David Edwards - Dentistry Microbiology Lecture series