How It Works: A Whole Answer on Half-Life

I had an interesting question the other day from a patient concerning the half-life of a medication. I’ll admit, I didn’t know the answer right off the top of my head, and I had to dig a bit back into my pharmacology 101 brain area to remember what I knew about the half-life of drugs so I could properly answer his question.

The half-life of a drug is basically the estimated time it would take for the concentration of it to be reduced by half. The half-life of a drug is not something that we in the regular outpatient clinical setting typically think about, because the dosages that we use have already been determined by research many times over. In fact, that is where our dosages come from. We want to maintain a certain level of the drug in the bloodstream for it to have the right effect, and we can alter it by either giving it different times per day, or by the total dosage per day. Half-life becomes more routine to think about when you are practicing medicine that involves toxicology, radiation therapy, and other types of acute care where medicines are very personalized per patient and will be monitored closely.

For most medications, it is thought that it takes about 5.5 half-lives to remove a medication from the body. Different substances have vastly different half-lives, so we have to do a little math. Let’s take Ambien for example. It is a sedative that is taken nightly to aid in sleep. It has a half-life of about 2 hours. That means every two hours, the concentration of the Ambien in your blood stream drops by half. So, you go from full dose to half dose, quarter dose, eighth dose, and so on every 2 hours. If the half-life is 2 hours, and it takes 5.5 half-lives to clear it, then it would take about 11 hours (2×5.5) to clear a single dose of Ambien from your body. That is why it is prescribed to be taken once nightly immediately before you lay down for bed, when you can expect to have 7-8 hours of uninterrupted sleep. We usually start at 5mg, because the 10mg dose may leave too much medication in your system by the next morning, leaving you groggy and potentially impairing your morning commute. You can also see why we want you to have 7-8 hours of sleep available. Don’t take Ambien when you have to get up in 4-5 hours, because ¼ to ½ of the medication is still circulating!

Notice that I said “about” a lot. Everyone metabolizes medications differently. Factors that influence half-life include age, sex, body composition, liver function, kidney function, genetics, and other medications. If you have poor kidney function, and you take a medication that is processed by the kidney, then you may have much higher concentrations of that medicine for longer. In cases like that, we actually have renal and hepatic dosing guidelines for most medications to guide us in dosage decision making. Overall it is our job to ensure safe and effective therapeutic drug levels for you.

I hope you enjoyed this blog on the half-life of drugs. I know it isn’t the typical stuff I write about, but I like to keep the topics centered around things that happen to me either here at the clinic or around town. As always, if you have any questions about this topic or anything else, or have a suggestion for something you would like me to write about, please come see me at 45 Urgent Care and I’ll be happy to talk to you.

-Jarrod Beachum, PA-C