How Long Does It Take for a Medication to Go Away When I Stop Taking It?
Here is the trick that makes this a story worth telling. Medications disappear more quickly when their concentration is high, and then more slowly when there is less of it around for the body to metabolize and remove. The result is a relatively smooth curve (of declining amounts in the blood) that falls steeply at first and very slowly at the very end ("exponential decay", if you know that term). We measure the disappearance of a meditation from the bloodstream using a concept of "half-life": the amount of time it takes for the amount of medication in your bloodstream to fall by one half. Hang on, this gets a little tricky.
It turns out that the rate of decrease is consistent, in a funny way. The amount of time it takes for the concentration (the amount of medication in a given amount of blood) to decrease by one half stays the same, even though the rate of decrease is fast at first and slow later, as shown in the graph below.
Here's how that works. Imagine that you have a medication in your bloodstream that starts out with 100 units in every milliliter of blood. When you stop taking it, your body continues to metabolize it, so the concentration is going to decrease. Let's say this is a medication that disappears quickly, like oxcarbazepine: in two hours, half of it is gone. So at two hours later, you have 50 units in every milliliter of blood. Now, because the liver will have a harder time finding those 50 units to remove (compared to when there were 100 units in every milliliter), the rate of disappearance slows dow. In two more hours (four hours from the beginning of this process), you'll be down to 25 units. Two hours later, you'll be down to 12.5 units. Two hours later, 7.25 units; then 3.125 units, and so forth. As you can see, the numbers very slowly approach zero. (This is called an "asymptotic" curve).
So, what should we use as the endpoint for all this? There is no obvious "zero", because the very end of this process takes a long time.
Instead, in medicine we use a standard assumption, which is not perfectly accurate but close enough. We say the medication is pretty much gone after "4 -5 half lives". If the medication has a half-life of 2 hours, then it will be close to gone in 8 to 10 hours after the last dose. Most medications have a half-life of about 24 hours, so they are gone -- or close to it -- in 4-5 days. A few medications have very long half-lives. Prozac, for example, takes almost a week to decrease by half, so even when it is no longer being swallowed, it takes over a month for it to be completely gone.
I'll bet the question you are really asking is how long does it take the effects of your medication to go away. Of course the changes in the brain chemistry that medications create do not immediately reverse themselves. It takes many more days -- perhaps weeks, or even months -- for the brain chemistry to go back to the way it was before the medication was started. So even though the medication goes away fairly quickly, the effects can last much longer than that. Sometimes people can stay well for months before relapsing. For example, imagine someone who has been stable for several years decides to stop taking her medication. Depending on which medication that was, and how well it was working, she might stay well for several months but then find her old symptoms coming back. On the other hand, those symptoms might come back more quickly if her medication caused brain chemistry changes that more quickly revert to their previous state.
In any case, you should not stop any medication without talking about it with your doctor. She or he can help you plan for what comes next, after that; and warn you about some of the bad things that can happen from stopping the medication suddenly. For many medications, especially if they are stopped suddenly, some very bad things can happen when they are stopped without some kind of supervision and backup plan. So make sure you DO NOT do this on your own.