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It is really important to get the blood level of a drug right. If it is too low the drug will not work. If it is too high there could be side effects.
For long term treatments over days we have to keep topping the dose up and so we take tablets "3 times a day" for example. But we have got to get the timing right or it is possible to overdose and the drug levels will start to build up. Just think of the bath example again. The aim is to get a fairly constant level of water within a high and a low limit by adding a bucketful of water at the appropriate times to counteract the loss through the leaking plug. This is the
If you keep adding a bucketful of water too often it will pour over the edge of the bath - even though the plug is leaking. This is similar to giving an overdose and getting side effects with a drug.
If you do not pour water in often enough the water level will fall too low for you to have a decent bath. This is similar to the drug not working - the level is too low.
What we have to do is get the timing right so that when the level is getting close to the low point we top it up again. That is why we take "tablets 3 times a day" for example. The ideal is to be able to take one tablet once a day. But that is a difficult goal to achieve. Our bodies are very good at getting rid of drugs from our blood stream.
This diagram shows what happens when we take 3 tablets. If we get the timing right the rate of addition of the second will exactly compensate for the rate of loss of the first and the resulting blood level will be steady.