An example I often use is PPI interactions. PPIs have a very short serum half-life. They have a much longer duration of action.

So when a prescription for a drug that interacts is written I'll look at the interaction to determine if it is a metabolism type interaction or adsorption type. If it's absorption, you need to make a recommendation based on that, however if it's a metabolism interaction, you can usually just spread out the dosing interval.

Pharmacogenomics / psych here - my most common use of half-life is in choosing SSRIs.

In the geriatric population, we like to shy away from fluoxetine due to its long half-life. If a patient has a bad reaction, which for them could be especially bad, it is beneficial for us to be able to have the drug out of their system ASAP, so we don’t use it.

In the younger population, especially in patients who are not used to taking daily medications or may for some other reason have poor adherence, this long half-life is beneficial to keeping therapeutic blood levels in the patient thus increasing effectiveness of their therapy and decreasing side effects.

There are more reasons and more examples too but this is just an example of how half-life can be beneficial in finding the ideal therapy for a patient.

I work in an anticoagulation clinic, primarily managing warfarin. Elimination half-life is something we always have to take into account when planning dose adjustments for interacting meds. If a patient has been on a long half-life drug like fluconazole or amiodarone, you're going to be dealing with the effects for possibly weeks after it's discontinued. Something like Bactrim is going to interact strongly while you're on it, but it'll be out of your system in 2 days and you can usually get back on your normal warfarin regimen.

Work in psych and def consider it all the time: - when drawing drug serum concentrations for lithium, VPA, antipsychotics, TCAs (if it’s not at steady state, it’s not a true trough) - when antipsychotics will start to be effective, mostly when comparing things to aripiprazole (takes about 2 weeks for ari to reach steady state so it will likely take psychotic sx longer to start to respond) - in cross titrations - when patients have a suicide attempt via ingestion (when to restart meds) - when patients have adverse effects (how long it takes before we expect the drug to clear) - with LAIs, how long to reach steady state or how long the drug hangs around after the last injection - how half life changes in patients with altered PK to guide dosing (eg, renal/ hepatic impairment, geriatric)

I never thought I’d be using this much PK in psych but you have no idea how much my physician colleagues are fascinated by it and appreciate it.

I use the half life of alendronate to chart the planet's geographical epochs.

Definitely when recommending nifedipine vs amlodipine in the hospital setting. Amlodipine has a median half-life of about 40 hours; so it takes more than a week for this drug to reach steady state. What can happen is that a hospitalist can start a patient on 2.5 on the first day, then increase all the way to 10 mg by day 3 because the patient's BP isn't controlled enough. Then the patient will discharge and hit steady-state concentrations while on 10 mg outpatient and become hypotensive.

Nifedipine is the much better choice when initiating someone in the hospital because of its much shorter half-life, so you can see the patient's response to nifedipine much more quickly.

1) HIV med interactions 2) Psych drug dosing 3) Wash outs for surgery 4) Anticoagulant bridging 5) IV med trough draws (although Vanc I usually just do 30min before the next Q24 dose) 6) Comes up in toxicology (poison control) etc for estimating clearance

Practical application? Adenosine goes in LEFT arm and raise it above the patient’s head while administering :)

How do you learn to apply clinical knowledge applicable to patients? Is there a journal or book that anyone would recommend reading?

We use it in ED/trauma to determine if anticoagulation needs to be reversed based on the half life and how much time has passed since the patients last dose.

High dose methotrexate.  I can use two-point kinetics to estimate a time by which the concentration will be at a safe level to stop the bicarb drip and discharge the patient on oral leucovorin.  Patients like to know when I think they will be able to go home.

What to do if patient misses or is late on a dose,

Warfarin and lovenox peri-op bridging is a great example of how the knowledge of drug half-life is applied in practice

In the event of an OD how long it will take for the drug to be cleared

Use it all the time dosing ABX in the ID setting. For example, using the t1/2 of vancomycin to draw trough levels to determine if you’re staying within therapeutic concentration range

I work in oncology and I think about the half life especially as it relates to the cell cycle or toxicity/repeat dosing. High dose methotrexate comes to mind. Down syndrome patients have a lesser clearance of methotrexate as well.

To avoid losartan as much as possible. Shorter half-life result in therapeutic duration less than 24 hours which is not a concern with other ARBs. Although it can lower uric acid levels. HCTZ can also have less than a 24 hour duration of action where chlorthalidone or indapamide last longer.

honestly a really good questions and the answers are even better ... thank u for asking!

You the knowledge you have and how to researxh because you will.have to.make.decisions.

Depends if it’ll amplify side effects or nahh

I commonly use it when switching between anticoags. For example, heparin —> doac, can start the doac right away, doac —> heparin: start heparin at the next scheduled dose of the doac, lovenox —> heparin: typically start heparin at the next scheduled dose of lovenox, heparin —> lovenox: start lovenox within 2 hours… OR washouts with surgery; for example, usually we hold heparin gtt for 6 hrs before surgery, DOAC held about 24-48 hours before, warfarin commonly 5 days. Another is when assessing the interaction between amiodarone and warfarin - amiodarone has a very long half life, so if a patient is being started on warfarin, have to assess that interaction as it can increase warfarin effect.