Over-the-counter diphenhydramine, for instance, at least in my country, says adults can take “1 to 2 tablets every 4 to 6 hours.”
If you decide “my symptoms aren’t so bad; I’ll just take one” and then two hours later your symptoms are still bad (or worse), is it safe to take a second tab then? And if you do, should you wait until “4 to 6 hours” after taking the first tablet or the second to take an additional tablet? Does it depend on the drug? (Maybe it’s fine for diphenhydramine but not for ibuprophen?)
I’d imagine blood levels of any particular drug tend to quickly spike and then exponentially decay back to undetectable levels. If you take two tabs, I’d imagine that graph is just twice as tall. If you wait a couple of hours between tabs, it’s got two spikes and the second is a little higher than the first (but not as high as the two-tabs-at-the-same-time spike.)
If the concern is total concentration of drug in the bloodstream at any one point, a second tab a couple hours later is less of a concern than two tabs at the same time. If the concern is total area under the curve, then probably there’s no difference between two tabs at the same time and a couple of hours between. If the concern is total time spent with a blood concentration of such-and-such, I could see there being more concern with taking a second tab just a couple of hours after the first.
And maybe there are other effects that I’m not aware of. Maybe if the blood concentration kicks up to two-tabs-at-once levels, the liver kicks into high gear, clearing the drug out quicker, but if you go a couple of hours between tabs, the liver neve kicks into high gear or some such.
And maybe this question hasn’t even been well studied and maybe there’s not really any good answer. But if there is, I’m curious.
There are reliable sources of drug information intended for professionals online, for example this AHSP monograph for pseudoephedrine. There’s a pharmacokinetics section with times to peak plasma concentration, half life, and a description of how the drug is eliminated from the body.
If you understand what those terms mean, you can probably make an educated guess as to what will happen when you split a dose as you propose. I’m going to guess that it’s really unlikely to be dangerous with most OTC drugs because those usually have large safety margins, but if you want a reliable answer to that, there’s probably a medical professional you could ask instead of random dogs on the internet.