Updated 3/2019

How long it takes for a medication to leave your body depends on its dose, and its metabolism.  But you may be asking how long it takes for the medication’s effects to go away.  That can take longer, sometimes much longer. Antidepressants are among the trickiest ones.  Here is the simple part: medications disappear from your body more quickly at first, and then more slowly when there is less of it around for the body to find and remove.

The concept of half-life: simple

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.  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 half.   This may sound complex but it’s actually pretty simple.

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.

illustrating medication metabolism
Medication leaving the body

Here’s how that works. Imagine that you have a medication in your body that starts out with 100 units in every drop 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 methylphenidate/Ritalin: in about three hours, half of it is gone. So at three hours after the last dose, you have 50 units in every drop 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 drop), the rate of disappearance slows down. In three more hours (six hours from the last dose), you’ll be down to 25 units. Three hours later, you’ll be down to 12.5 units. Three hours later, 7.25 units; then 3.125 units, and so forth. As you can see, the numbers very slowly approach zero.

So, when has all the medication left your body? There is no obvious “zero”, because the very end of this process takes a long time.

Using half-life in medicine

Instead, in medicine we make an assumption. It’s not perfectly accurate but close enough. We say the medication is pretty much gone after “4 to 5 half lives”. If the medication has a half-life of 3 hours, then it will be close to gone in 12 to 15 hours after the last dose.  But notice that half was gone in 3 hours, and by 6 hours you were down to a quarter of the original highest amount. The medication effects could be gone even though some medication is still in your blood.

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. Fluoxetine/Prozac, for example, takes almost a week to decrease by half, so even when it is no longer being swallowed, it takes slightly over a month for it to be completely gone.

But when is the medication effect gone?

I’m guessing 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. By then, it’s not so clear if stopping the medication was really the culprit.

How long for antidepressant effects to go away?

My area of interest, as you’ll see throughout this website, is the spectrum of mood disorders, from plain depression to “bipolar”. Antidepressants can do really weird things in some people, even those in the middle of the mood spectrum who don’t look “bipolar”. They can cause agitation, irritability, insomnia, and more.

Ironically, antidepressants can have these weird effects when they are started, or months after they’ve started — but also when they are stopped.  And unfortunately, for some people the effects of stopping an antidepressant can last for weeks, or months. This is not a half-life issue, it’s a brain effects issue.  If that’s why you landed here, I’m sorry; and I’d urge you to learn more on my antidepressant withdrawal page.

A reminder

In any case , you should not stop any medication without talking about it with your doctor or nurse practitioner. She or he can help you plan for what comes next.  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.