Compounding, or manipulating, a drug is likely to change the drug's behavior in the body, even if used in the same species for which the drug is approved. Most of the changes will either alter the stability of the drug in its preparation or its absorption. Generally, the more a drug preparation is manipulated beyond its original dosing form or route of administration, the more likely the changes will negatively impact the drug's performance in the animal's body.
Transdermal drug delivery using a gel offers a good example. Although this route and method of administration offers tempting benefits because of its convenience, absorption thus far has been demonstrated only for limited drugs or conditions. Although the most likely reason that drugs administered as a transdermal gel do not reach systemic circulation is failure of the drug to penetrate the stratum corneum. Issues such as drug stability, altered ionization, drug dissolution and accuracy in dosing may also contribute to poor drug delivery with this type of delivery. On the other hand, many transdermally administered drugs can depot in the stratum corneum after multiple doses, and will begin releasing into systemic circulation at doses much higher than originally intended. This can result in toxicoses. Veterinarians should not assume that any compounded drug preparation will result in drug delivery that is equivalent to an approved version.
Veterinarians should not interpret the availability of a compounded product as evidence of safety or efficacy. The pharmacist's responsibility to the patient and clinician is to prepare a drug according to the veterinarian's prescription using the National Association of Boards of Pharmacy's Good Compounding Practices. It is the responsibility of the veterinarian to determine if a prescribed compounded preparation is likely to be safe and efficacious for the patient. While properly trained pharmacists may achieve a high quality and safe product, there is no approval process for compounded drugs and as such, effective drug delivery cannot be guaranteed. Prior to using a compounded drug in a specific patient, good communication between veterinarian and pharmacist is constructive, and may include exchange of the scientific literature and determination of objective assessment parameters to determine efficacy, toxicity or failure of compounded medications.