Crossing over use of human health care medicines to animals does happen especially when treating companion animals. Table 3 shows examples of products originally formulated for use in humans that have crossed over and are also now being used in animals.
Injectable products are generally good dosage forms for (almost) direct transition to use in animals. If dosing volumes are acceptable for the animals being treated, the direct use in animals can be pursued. Immediate-release tablets appear attractive candidates, but might prove difficult, particularly if there are palatability issues with the product. This can be overcome by instructions for pushed administration down the animal's throat or to hiding it in food or treats. Even so, it should be realized that the shorter gastrointestinal tract transit time for dogs and cats could result in inadequate absorption. Also, in both cases (injectables or oral immediate-release tablets) there may be different extents of drug metabolism or different drug absorption mechanisms in animals compared with that in humans; so these aspects need to be considered and examined. A good understanding of pharmacokinetic parameters in the different species is important when attempting to cross over human medicine to animals. Other dosage forms like topical delivery systems can be even more problematic since the transdermal absorption process will change depending on the skin thickness and components, amount of hair, and the solvent and penetration enhancers used. Similarly, ocular formulations, oral films, and oral controlled-release tablets need to be studied carefully if a switch from use in humans to animals is desired.
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