Simplifying Pharmacokinetics, Why do I Have to Take This Medication Four Times per Day?

Pharmacokinetics is a scary word for many in the medical field.  The best way that I remember being taught pharmacokinetics is to think about what the body does to the drug.  If we understand pharmacokinetics, we can definitely optimize our patient’s medications and also help avoid unnecessary adverse effects and toxicity.

Half-life is one of the cornerstones of pharmacokinetics.  The half-life is a pretty simple concept.  Half-life is the amount of time that it takes for “half” the drug to be eliminated from the body.  Many times (but not always), we can get a sense for the half-life of a drug by how many times per day the medication is given.  Let’s use an example:  Hydralazine is a medication that can be used for hypertension.  I don’t see it used very often.  One of the major reasons I don’t see it used very often is patient adherence.  It is significant burden to ask our patients to take a medication four times daily.  The half-life of hydralazine is short.  Per Lexi-comp, with normal kidney function, it is about 2-8 hours.

Another classic example is antibiotics.  Azithromycin (Zithromax) has a long half-life, so we can use once daily dosing versus Keflex which has a very short half-life so we need to give multiple doses.  Antibiotic therapy does get a little more complex than this as some medications are concentration dependent killers and some are time-dependent killers.

Summing this concept up, we can’t always predict half-life from frequency of dosing, but in many cases it can be helpful to give you a sense of how quickly the drug is eliminated from the body.

Here’s a case where we can use pharmacokinetics to our advantage!

Written By Eric Christianson

June 17, 2015

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