What does the body do to the medications? (also called kinetics)

Once we take a medication many things happen. The medication needs to get to the body (absorption), it needs to be transported to the brain to exert its function (transportation) and then needs to be eliminated from the body so it does not intoxicate the person (elimination).

Absorption: AEDs can be taken in different formulations.   The amount that is absorbed will vary depending on the route it took to get into the body:
• By mouth: it is very important to know which medications must be taken with food and which ones must not. It is also important to know if specific foods need to be avoided with certain medications.
• By skin (patch)
• Intravenous: absorption is 100%.
• Intramuscular
• Rectal

Transport: once absorbed, medications need to travel to the brain which is where they will exert their function.
• Getting to the brain to get to the brain, the medication needs to travel (to be transported). Some medications go by themselves and some will be taken by “transportation” (proteins). This is called “protein binding” (the amount of medication transported by the protein of the blood). Some AEDs will have high protein binding, some low and some none. The reason it is important to know this is because if a patient is taking many medications that travel through proteins, there might not be enough proteins for all and some medications may get displaced. When the medication is displaced they become less protein bound and there is more free medication in the system. Why is that important?
• In order for the medications to get to the brain so that they can accomplish their mission of stopping seizures, they need to cross a barrier (called the blood-brain barrier). Only small particles can cross into the brain and proteins are not small. So the medication that travels with proteins cannot cross, only the ones that do not travel with proteins (free fractions) can do this.
 
Elimination: once the medications enter the body, the body starts working to eliminate them. This is important in order to “clean” the body for toxic compounds.  There are two main places where medications are eliminated:
• Liver: this is the body’s “laboratory”. Most medications, but not all, are transformed and sometimes inactivated by the liver. This results in the medications leaving the body before they can cause harm to the patient. Some medications (“inducers”) make the liver work faster (e.g. Tegretol, Dilantin, Phenobarbital) and some slower (“inhibitors”) such as Depakote.
• Kidneys: some medications do not go through the liver.  Instead they are eliminated only by the kidneys. An example is Keppra.