A little bit of pharmacology: Why do we have to take one medicine every six hours and another one twice a day?

In a previous post I defined what pharmacology is and explored what a dose-response relationship is.

Here’s a little bit more of pharmacology.

We are all familiar with the various ways in which medications can enter the body: orally, injected, ointments, etc. we are also familiar with the fact that medicines need to be taken periodically at various time intervals, depending on the specific medication.

The questions is “Why?”.

Well, it has to do with something called “pharmacokinetics”. This term simply means the path that a drug takes as it enters your body, gets transported throughout your system, while it is “processed” and when it exits your body.

Mind you, as far a medications go, just like The Doctor (yes, I am a Whovian) what we start with may not look like what we finish with; meaning that the nature of a drug can be changed by your body.

Pharmacokinetics is controlled by four main parameters:


ADME for short. Visually, it can be represented like this:
Credit: Baldscientist

Annnnnnd now you know why I am a scientist and not an artist. By the way, did you know that the word “scientist” did not even exist until the 1800s? The term was actually suggested in analogy of the word “artist”. I actually mentioned this story in chapter 1 of The First Brain…


Credit: vsplanet.com

Yes, I am like that.

Anyway, whenever a drug is given regardless of the route of administration, it will be distributed throughout the body, mainly by the circulatory system and when the drug finds its particular target, it binds to it depending on its affinity (another concept that we will explore in a future post). Upon distribution, over time, the drug reaches a maximal concentration within tissues depending on the amount administered, while at the same time the metabolism (usually by the liver, but in principle all cells can metabolize) and elimination (usually the kidneys, but it can also happen through other routes) processes keep working. There will be a point where the concentration of the drug will go down until it is completely eliminated. Graphically, it looks like this:
Credit: Baldscientist

For most drugs to work, it needs to be present at a more or less constant concentration so it will do what is supposed to. This constant concentration is called the steady state, which simply means that the rate of elimination pretty much equals the rate of elimination. The trick is to give the drug at specific intervals in such a way that the next dose goes in before the previous one is eliminated. It looks like this:
Credit: Baldscientist

Since the period of time that goes from absorption to complete elimination of a drug depends on the specific drug, this makes a difference in the time between dosages, i.e., every six hours, every 8 hours, etc., as shown above.

By the way, a much better drawn figure can be found here.

So there you have it. This is of course a simplified version of events. In future posts we’ll touch upon additional aspects of the effect and fate of drugs in a body, from the half-life to side effects. Stay tuned!

Credit: Baldscientist


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    1. Not really; pharmacodynamics deals with the characterization of drug targets (receptors, enzymes, etc). By its nature there is quite a bit of information about it and you need to consider each type of target individually, whereas the same pkinetics principles can be applied to any drug… i am planning to tackle pdyn in a future post… Hope this helps and thanks for the comment!


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