Why are medication times always in "whole"hours?

A disclaimer: this does not have to do with any medications I am on or anyone I know is on. I’m just curious.

Let’s say someone is on a painkiller he is supposed to take every four hours. How is it that medications are always taken at a whole number of hours–4 or 6 or 8 or 12, and not at, say, 3 1/2 or 8 and a quarter? I assume it’s because it would be too hard for some people to calculate when to take their next dose. Yet don’t these medications have half-lives? If the patient takes the painkiller every four hours, wouldn’t be likely the painkiller lasts for a shorter time (say, 3:25 minutes) or a longer time than exactly four hours? (Notice, too, that it’s always an even number, never every five or seven hours.) If so, it would seem the patient would either be taking the pain meds after the first dose started wearing off off or would be slightly more swacked from taking doses too close together, which doesn’t seem to be the case, in my limited experience. It seems unlikely they’d ALL last an exactly even number of hours.

Do all drugs that have to be taken at a set time wear off at that exact time, or do many of them wear off at an odd number of minutes? Are they all time-release? Also, do these drugs (whichever ones they are), START wearing off at, say four hours, or have they completely worn off by four hours?

Thanks for any and all information!

And why the same dose for a 95 lb women who is 4’ 11" as for the 6’ 4" guy who is 295 lbs?

Just sayin’.


There is no exact time. Phrasing the answer as generally as the question, the effects would vary from patient to patient depending on a range of factors. Further, the effect on a given patient would not cease abruptly at any particular time but would decrease gradually over a period. Consequently, the times given are rounded off best approximations. They would be based on ranges found in testing, with no doubt safety factors and so on added in, resulting in a number that would then be rounded off to something easy to remember and follow.

4, 6, 8 and 12 all divide evenly into 24. 5 and 7 do not. This is not a coincidence. It’s all about distributing the drug delivery over the course of one day. If the drug companies wanted, they could make a pill for each strength that was time-release, and you could take one of them. That pill would cost significantly more, though.

You can also work with your doctor to tweak your timing - I have one med that is normally an 8 hour dose, but I seem to burn through it in 6 hours, so the scrip is written for 4 times per day instead of 3 times per day and the number of pills adjusted accordingly. The insurance company whinges about it every damned time but the way the scrip is written makes them unable to contest it.

How could they make a general dosage description that could cover all of these things for all patients so precisely? In any case, even the doctor isn’t going to know (or care that much about getting) the exact number of parts per ml of the chemical in your blood stream at all times. Most medications are tested and manufactured to work at general levels.

If you’re that concerned about these minor points maybe you should just use an IV drip for everything–good luck finding one for aspirin, though.

Most medicines have a wide therapeutic range (from the minimum effective dose to the maximum tolerated dose). Thus a higher dose can be used to provide a longer time between doses. For example, 125 mg of amoxicillin is enough to kill susceptible bacteria in the urine, but you could take grams of it before you would notice toxicity.
Most of the time we use antibiotic doses that are several times the minimum effective dose. If you look up doses for children you find that we use the same dose for a 40 kilo child as for a 100 kilo adult. Lucky for us, most drugs are well tolerated. For those few with narrow ranges, we must calculate carefully and often check blood levels.
With many drugs we use the patient as our test subject and start with a low dose and increase until the desired effect is obtained.

They usually should start their activation not inside the dry, cool pill bottle or blister, but when exposed to heat and moisture (and stomach acid) inside the human body, which dissolves the outer covering to let the active ingredient get into the blood stream. (Except for other forms of taking medicine, like dermal patch, injection, anally, into the vein…)

Because they have to write the instructions for millions of people with different body chemistry. They can’t in advance figure out how you, Romeo, react to ingredient XY and how quickly your body metabolizes it and say “Romeo needs to take the pain pill every 3.25 hrs., but Julia has a slower metabolization and needs to take them every 4.75 hrs.”, so they go for a middle point that hits roughly.

Also, with pain med. esp. (or fever, sore throat) many doctors recommend to take as often as you feel like needing them.

Depends completly on the drug. Some are taken every four hours because it takes that long for the active part to get to where in the body it’s needed. Some others are time-release on purpose, so the active part is released over 12 hours - this avoids a spike in the first few hours and low levels later.

Not generally. Either your doctor calculates it and tells you “1 1/2 pills every 4 hrs” or the leaflet has a table how often for men, women and children, and how many mg. active per kg of body weight, given that one pill has x mg active.

However, there is a genuine concern there, that doctors slowly have started to address with women, and don’t know what to do about with children:

Women have different bodies from men in terms of hormones, which affect how medication is metabolized and what effects it has. Until recently, recommendations for women were just calculated according to lower statistical body weight, but now they are starting to research that a different mixture, a different medication because of side effects / long term effects etc. might be better when treating women instead of men. The researchers complain that this area is getting hardly any attention, and esp. not enough funding.

With children, it’s even more difficult: their organisms are different on a biochemical and metabolical level, too - but whereas with women, you can at least ask for volunteers for tests, the same is forbidden for children, obviously. (And no, at least in Germany, parents are not allowed to volunteer for them). So doctors have to figure out what would be better working only with blood and tissue samples or computer models.

Wanted to add: one dream of many doctors is that in the future, if we have each patients full genome and if we can get a full understanding of their biochemistry from that (both big ifs), then we could tailor-make medication for each patient -not for aspirin, but for serious stuff like cancer chemotherapy.

Some meds are dosed to your weight, generally as a mg/kg dose; cyclosporine comes to mind as one. For others, “close enough” may be fine, or as someone else stated in a recent thread, just because you have different weights doesn’t necessarily mean that you have a different amount of pain receptors, etc.

I think the way someone phrased it in another thread was that a 90-lb woman and a 300-lb man have about the same size brain, which is where a lot of these drugs work anyway.