The skin is one of the few organs to which drugs can be applied directly. This is the usual way of treating ear problems in small animals and foot problems in large animals. Many drugs which are applied to the skin have already been covered, the purpose of this chapter is to tidy up the loose ends.
Many drug formulations are available for topical use on the skin of animals. In general, surface active drugs can be classified as either mechanical or chemical in their mechanism of action. Thousands of drugs have been applied to the skin over the years; their use is largely empirical. The usual intention is for them to act locally, but to reach the deeper layers of the skin, they must penetrate the stratum corneum - many drugs carry on in and are aborbed systemically. Animals often lick drugs off the skin, again leading to systemic absorption. This can occasionally be useful - sometimes drugs are applied to the flanks of cows for them to lick off, but usually it is undesirable and can lead to toxic doses being absorbed.
To understand what happens when drugs are applied to the skin, you must know some histology and pathophysiology of the skin - absorption, and thus clinical effects, can vary enormously.
Remember that all drugs formulated to cross animal skin will affect human skin too - warn the owner to take care, or better still, give them some gloves. This is not absolute protection since drugs such as DMSO will happily diffuse through ordinary rubber gloves as well as skin, but is sufficient for most drugs.
