Q: How is medication absorbed once a horse has consumed it orally?
A: Shortly after a horse has been given some medication by mouth, the drug travels through the oesophagus, into the stomach and is then (predominantly) absorbed into the bloodstream through the lining of the small intestine. The level of the drug in the bloodstream gradually rises as it is absorbed, and then falls as it leaves the circulation to enter the target site, for example, a joint or the lungs.
The blood level of the drug must be maintained above a minimum level to ensure an adequate amount of drug is always being delivered to the target site. If the target is inflamed (as it often is) it will receive an increased blood supply, so more drug will be supplied to it than any other tissue in the body. However, blood is also travelling through the kidneys and they will identify the drug as ‘foreign’ to the body and work to remove it from the circulation by passing it into the urine.
The dose rate of the medicine prescribed is calculated so that an adequate level remains in the blood, despite the action of the kidneys, to deliver sufficient drug to the target tissue until it is time for the next dose to be administered.
NOTE: Partial consumption of the medicine will not maintain the blood level high enough for long enough, so the drug will be ineffective.