Question about medication dose frequency

[w0mp]

---

Members don't see this ad.

This seems like a very basic pharmacology question, but I can't seem to find a practical answer for it:

What is the general strategy for the dosing frequency of a medication?

I have a feeling it must depend on its half-life and/or peak onset time, but for all the medication examples I have found, there doesn't appear to be a consistent relationship. For example: cephalexin has a half-life of 0.5-1.2 hrs and peak plasma time of 1-2 hrs but it's dosed q6h. Oxycodone has a half-life of 2-4 hr and peak plasma time of 1.5-1.9 hrs but can be dosed anywhere from q4h to q12h.

Are there any general rules or guidelines that you could share with me? I know I can look up on the pharmacopoeia how to dose a medication, but I would like to know the calculations behind it.

 

[mvenus929]

I imagine it has something to do with the level needed in the blood/tissue to maintain an effect (just because the half life is 1.2 hours, doesn't mean that it stops having an effect when half the drug is metabolized, depending on the dose). If 500 mg of Flagyl causes blood levels to stay high enough 5 hours later to have an effect, then you redose when the level drops below the needed level. I imagine it's a bit different for each drug based on toxicity profiles and therapeutic window.

 

[Cytarabine]

Your goal is to hit therapeutic concentration. Let's say that's 5-10. If you dose continuously (i.e. IV drip), you'll be able to reach a steady concentration of 7. If you give an IV bolus every 4 hours, you will have some oscillation in serum concentration, i.e. 6-8. If you space that dosing out more, that oscillation might go to 3-14. You have some therapeutic time, but you also have subtherapeutic and supratherapeutic (and potentially toxic)

That's kind of bare bones. Varies based on formulation (ER tabs), some meds are effective based on their peak serum concentration and are dosed accordingly, etc

 

[link2swim06]

It is very medication dependent. Half-life is only one piece of information.

You need to consider things like:

-action/mechanism of medication: is there a certain level you are trying to maintain above or does it even matter if the level fluctuates?
-distribution (likes to stay in blood, tissues, etc.): will it go straight into the fat (i.e. propofol) and then linger for a long time
-kidney/liver function: if the person is on dialysis it may not go away until they get their next dialysis session
-steady state concentrations: you almost never have no drug left when you get a repeat dose...after 5 doses the stacking will lead to an approx. 90% steady state concentration

You could read literally multiple textbooks on how antibiotics work (bactericidal vs bacterial-static) and the madness behind their dosing.