[ELI5] Why is there, or is there, a need to wait 6 hours between medication?
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Medication hangs around your body for a while. The exact dynamics can be very complicated (it’s a whole subfield of medicine called *pharmacokinetics* – try saying that, it’s fun), but as a simple approximation, we can imagine a medication that follows a very simple rule. That rule is that each hour, every molecule has a 1-in-5 chance of being broken down. (This is an example of what’s technically called *first-order kinetics*, meaning that the rate of breakdown is proportional to the amount of the drug in your body.)
So, you wake up, and at 8 AM, you take 100 mg of this medication (we’ll assume this medication is quickly absorbed, e.g. under the tongue). In the next hour, 1 in 5 of the molecules breaks down. You now have 80 mg left in your system at 9 AM. In the hour after that, 1 in 5 of the remaining molecules breaks down, which leaves you with 64 mg in your system (since 1 in 5 of the 80 mg at the start of that hour broke down) at 10 AM. And it keeps breaking down:
* 11 AM = 51.2 mg
* 12 PM = 40.1 mg
* 1 PM = 33.8 mg
* 2 PM = 26.2 mg
* 3 PM = 21.0 mg
* 4 PM = 16.8 mg
And so on.
Now, imagine this medication does its job if you have at least 20 mg of it in your body. But having more than 150 mg will cause an overdose that harms your body. That means that you want to take another dose of 100 mg *before* you drop below 20 mg, but *after* you’ve dropped below 50 mg (because if you take it while you have, say, 60 mg, a 100 mg dose will spike you to 160).
So you need to take it *after* 11 AM (because you still had >50 mg left at 11 AM) but *before* 4 PM (when you’ve dropped below 20 mg). So you might, if you were a doctor, say “take this medication every 6 hours”, so that if you take it at 8 AM, you take your next dose at 2 PM, neatly in the middle of that window (which allows for the fact that you are probably not sitting around with a stopwatch, although there are a few medications that practically do require you to do that).
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There’s a few things that affect the length of this window and how important it is, namely:
Some drugs have narrower ranges of effectiveness-vs-safety (that is, narrow [therapeutic windows](https://en.wikipedia.org/wiki/Therapeutic_index)). The medication above has a relatively window, from 20 mg to 150 (that is, you overdose on 7.5x the effective dose), so it’s important to keep to a regular rhythm.
Some drugs can tolerate toggling off and on. Others aren’t effective if they’re toggling off and on (say, because they’re suppressing some process, and if they drop too low that process can start up). A pain medication can usually tolerate some toggling, whereas a heart medication probably can’t.
And different drugs break down at different rates. Typical medications typically have half-lives (the time it takes for them to drop to half their initial value) of somewhere between an hour and a day or two, but there are many, *many* outliers; I am on a regular medication with a half-life of approximately a week so it’s not very important when I take it (because the levels stay pretty flat anyway).
They can even break down in different ways: some drugs break down at a constant rate no matter how much you have (“zeroth order kinetics”) and hit near zero not long after getting low. Others have complicated interactions where the breakdown depends on the amount of the drug *and of some other chemical* (“second order kinetics”), which can be *very* complicated for a pharmacist to handle. Worse yet, the rate can be affected by lots of details of your body, like liver or kidney function or other medications you’re taking or even certain foods (grapefruit, famously, stops many medications from breaking down by blocking a liver enzyme).
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