Medications undergo a process called biotransformation. For example:
- An active drug such as phenobarbital is converted, prior to excretion, to an inactive metabolite: hydroxyphenobarbital.
- An active drug such as primidone is converted, prior to excretion, to an active metabolite: phenobarbital.
- An inactive drug such as codeine is converted, prior to excretion, to an active metabolite: morphine.
It is either the active drug, or the active metabolite of the drug, that actually does the job it was intended to do, in the body.
Drugs eventually need to leave the body and the majority do so through the kidneys in urine. However, other drugs are excreted by the liver in bile .
Water soluble drugs are excreted through the kidneys either unchanged (ampicillin), or having been changed in the liver first (Rimadyl, Novox).
Other drugs pass through the liver and are excreted from the liver in bile. Bile enters the digestive tract and the drugs are eliminated via the feces. Examples are: ketoconazole (Nizoral).
As you can see the liver and kidneys play a major role. That's why it's important to make sure that these organs are functioning properly. In some cases, blood tests need to be performed before beginning drug therapy, and depending on the drug, during therapy to make sure there is no damage being done to the liver or kidney. When giving the pet more than one medication, kidney and liver health becomes even more important.
I'll try and expand on this subject in future entries.
