Folks often ask questions about their medicines (whether over-the-counter or prescription) that have to do with how long they last, or how long it will be before they take effect, or how long until side-effects may go away or for the medicine to be out of circulation.
A handy principle or "rule of thumb" has to do with what happens to medicines after you take them; this is known as pharmacokinetics. (On the other hand, what they do after you take them is called pharmacodynamics.) Mind you, this rule of thumb only applies when a medicine is being taken regularly and as directed. This means that it is being taken in the instructed amount and frequency and not "as needed" or "now and again". However, this rule of thumb applies to nearly any medicine by any route (swallowed, injected, i.v., etc.).
How much of a medicine (dosage) and how often it needs to be taken (frequency) has to do with how quickly your body metabolizes or breaks it down, and then how quickly it is excreted out of your body. For the vast majority of medicines, the liver breaks them down, and then they are excreted in your urine.
When taken at the directed dose and frequency, a medicine does not stay at a fairly constant and effective level in your blood stream until after the 4th dose. This is called steady state.
For example, if today you start taking a pill for pain that is supposed to be taken three times every day, then a steady and pain-relieving amount will start to be maintained in your system by later tomorrow. On the other hand, if you start one that is a once a day pain pill, it will not reach a steady level until five days from now.
Similarly, if you are taking a medicine regularly and feel that it is causing troublesome side effects it will not be entirely gone until after the fourth missed dose.
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