Topical skin treatment has a huge advantage over systemic therapy. The drug can go direct to the part of the skin that is diseased so high concentrations can be used with minimal risk. Cardiac drugs can harm the skin, but topical skin drugs do not harm the heart!

What Influences Absorption?

Body site: the palm has very thick stratum corneum so absorbs little. The scrotum has thin stratum corneum so absorbs a great deal.
Stratum corneum function: if the stratum corneum is moist (e.g. in body folds) much more will be absorbed than in dry exposed areas.
Influence of the disease: if the stratum corneum is lost or diseased absorption is easier. So in treating psoriasis the drug gets in more easily at the diseased sites than the surrounding normal skin.
Size of what is being absorbed: if the molecule is too large, very little will get in. Penetration enhancers such as propylene glycol may be added to topical drugs to increase drug absorption.
Most topical drugs never get into the skin but end up on clothes.
All topical drugs that get into the skin are eventually absorbed into the circulation, but the quantity is usually very small.

What Is a Cream?

A cream is a semi-solid mixture of water and lipids. Creams usually look opaque white (like fresh cream). Water and lipids do not blend, but can form an ‘emulsion’ in which small droplets of one are suspended in the other: either droplets of lipids in water or droplets of water in lipids. If the cream is ‘lipids in water’ (e.g. aqueous cream), it will evaporate and so is cooling, and the cream will mix with water so it can be washed off. If the cream is ‘water in lipids’ (e.g. oily cream), it is more difficult to wash off. Because creams contain water, they can spread easily over moist areas of diseased skin whereas ointments slip off.