Pharmacokinetics Explained



This line represents a whole pharmacokinetic aspect of what happens when a single dose of a medicine is given. You see on the left hand side plasma concentrations and on the bottom against time. Time will depend on whatever it is, whether that's hours, days, sometimes weeks. I've used arbitrary figures. And you can see a curve there and I've coloured it to indicate the area under the curve. As you can see when you get to the 0 point (in the bottom left-hand corner) you see at this point in time there is a 0 blood level of the drug (because you haven't taken it yet.) And when you take it, within a certain period of time you see there's increase in plasma concentration of the drug and that is called the absorption curve, and many, many factors affect that. It gets to a point called the peak height and then it starts to come down and this is called the elimination curve. In the centre of that curve you see distribution. So, during that time the drug is being distributed throughout the body. It might be going through the liver, it might be metabolised to some extent. And you see all the area under the curve or AUC. For virtually every medication or drug that we take, there is a minimum effective concentration. That is the line between labels of 20 and 40 - about 30 there - that is the part where you start to get an effect of the medication. Then when you get to the top bit there is another line there that is where you get to toxic concentration. So, what we have to do is to allow the dose - to be able to - for the drug to fit in within that dose range, and that is known as the therapeutic range. For those of you American text, it's 'therapeutic window' (exactly the same thing). At the bottom of the graph you can see there's an arrow pointed to what is 'onset'. The onset of action is when the absorption curve crosses that minimum effective concentration line. A bit further down you see the 'termination' (or 'offset' sometimes it's called) and that is when there's not enough medication left for it to have an effect on you. In other words, that is the time that the medication will be effective. The time between onset and termination is the length of time that that medication is going to at least have some pharmacological action.
CPD time2m
First Published04 September 2019
Updated03 September 2019
30 August 2027
Learning Tools