There are two organs, the liver and the kidneys, that serve to eliminate (metabolize) the majority of medications, and other substances, from the body. Without going into painful detail about how they do this, here are a couple of useful concepts.
You can use the metaphor of a highway when discussing how medications exit the body. If you think about having a certain number of “off ramps,” it’s easy to see how, at rush hour, these become overwhelmed and the highway backs up until you’ve got a traffic jam. The majority of drug interactions occur because medications either compete for the same off ramp or influence the number of available off ramps.
In the liver there are systems of enzymes (proteins) that break down medication and other substances. The production of these enzymes can be increased or decreased by many different substances (up regulation and down regulation). Continuing the metaphor we can see how if drug A needs a particular off ramp and drug B has either tied up that off ramp or shut it down, drug A is going to back up in the system.
The number of medications that inhibit or induce enzymes in the liver is staggering. Most physicians and pharmacists now use software to run drug-to-drug interaction profiles when prescribing a new medication. It’s always a good idea to ask, “Will this new medication interact with anything I’m already on?”
It’s important to remember that not only do prescribed medications cause drug interactions, but so too do over-the-counter drugs, herbal and nutritional supplements, and even some common foods and beverages, grapefruit juice being one of the most frequently cited examples. Cigarette smoking is also associated with increasing the elimination of many antipsychotic medications, in some instances reducing the blood level by half.