Pharmacology Basics, part 4: Therapeutic Index, Half-life, Medication Mode of Action

by Cathy Parkes August 09, 2021 Updated: September 13, 2021

In this article, we'll explain some terms you'll see often when learning pharmacology—the therapeutic index, half-life, and mode of action. You'll also learn the difference between an agonist, antagonist, and an agonist-antagonist or partial agonist.

This series follows along with our Pharmacology Basics and Safe Medication Administration Flashcards for Nursing Students which are intended to help RN and PN nursing students build a strong foundation going into Pharmacology and as preparation for the ATI, HESI, and NCLEX.

Therapeutic index (TI)

The therapeutic index for a medication is a range of doses at which the medication is effective without adverse toxic effects. The low end of the therapeutic index range is the minimum amount of a drug needed in the body to be effective (minimum effective concentration). The high end of the therapeutic index range is the maximum amount of that drug that should be in the body (beyond which is toxic).

Medications with a higher therapeutic index are safer because there is less likelihood of reaching the toxicity amount, so there is no need for close monitoring of blood levels. A wide range of drug concentrations would be okay. One example of this is the antibiotic amoxicillin.

Medications with a low therapeutic index require close monitoring of blood levels since the toxic amount of the drug can be reached too easily.

Only a narrow range of drug concentrations is safe, so this one is more difficult to hit exactly. One example of a low therapeutic index drug is the antibiotic vancomycin.

Trough levels should be taken immediately before the next medication dose.

What are trough levels?

Trough levels are the drug concentration levels the body reaches immediately before the next dose is administered. The word trough refers to the concept of a graph of drug concentration. Drug concentration in the body forms its lowest point (which looks like a trough on the graph) before the dose is administered again.

Half-life

The half-life of a medication is the amount of time it takes for that medication to be reduced by 50% in the body. Medications with a short half-life will leave the body quickly. On the other hand, medications with a long half-life will leave the body more slowly, and therefore, carry an increased risk for toxicity.

Medication mode of action

A medication's mode of action, or mechanism of action, is what the chemicals of the drug actually do in the body. Once the medication is absorbed, how does it act?

In our Pharmacology Flashcards we explain an easy-to-understand mode of action for every medication that we cover, as you need to know then in nursing! Medications act differently depending on whether they are an agonist, antagonist, or partial agonist.

Agonist

Agonists activate a receptor in the body. For example, dilaudid is an opioid agonist, and it activates the opioid receptors in the body which produce analgesia (pain relief).

Antagonist

Antagonists will block receptors in the body. For example naloxone, which is an opioid antagonist (and the antidote to an opioid overdose). Naloxone blocks the aforementioned opioid receptors, which reverses the analgesic effect of those opioid medications.

Agonist-antagonist (partial agonist)

Occasionally, you may see a medication that is an agonist-antagonist—which is both of those things. This type of medication is also called a partial agonist. Agonists-antagonists act as an agonist/partial agonist at some receptors, and as an antagonist at other receptors.

For example, buprenorphine is an opioid agonist-antagonist that has a lower risk for dependence and abuse than opioid agonists.

Buprenorphine, naloxone, and dilaudid are all covered in the nervous system section of our Pharmacology Flashcards for Nursing Students. Our Pharm Basics series gives an overview of the basics that you need to know in order to dive in and be successful in pharmacology.


Full Transcript

Hi. I'm Cathy with Level Up RN. In this video, we are going to talk about therapeutic index as well as half-life and the mode of action of medications. At the end of the video, I'll give you guys a little, simple quiz to see how well you picked up on some of the key concepts that I'll be covering. Won't be anything too complicated, like a "select all that apply" or anything like that. It'll be straightforward.

So let's first talk about therapeutic index. Therapeutic index or TI compares the minimum effective concentration, which means the amount of a drug we need in the body to be effective - right? - to give that therapeutic effect, and it compares that MEC to the level at which the drug is toxic, and we don't want to go over this amount. We need to hit somewhere in between these two levels. So a drug that has a high TI, a high therapeutic index, that means we have a lot of room between this MEC, this minimum amount, and this level at which it's toxic. So it's pretty easy to hit somewhere in this range. That means the drug is safer, and there's no need for close monitoring of the patient's blood levels. So an example of a drug with a high TI would be amoxicillin. However, if we have a drug that has a low TI, a low therapeutic index, that means that the minimum effective concentration is pretty close to the level at which the drug is toxic. So we have this really narrow range that we need to hit, and that means that the drug has a higher risk for toxicity because it's hard to just nail it right in that small range than it was when we had that drug with the high therapeutic index. So an example of a medication that has a low therapeutic index is vancomycin, and it will require close monitoring of the patient's blood levels.

All right. Next, let's talk about half-life. Half-life is the amount of time it takes for a medication to be reduced by 50% in the body. So medications with a short half-life will leave the body quickly. On the other hand, medications with a long half-life will leave the body more slowly, and therefore, there is increased risk for toxicity with these medications.

Next, let's talk about the mode of action of medications. Some medications act as agonists.

Agonists will activate a receptor in the body. So for example, Dilaudid is an opioid agonist, and it activates the opioid receptors in the body which produces analgesia.

Then we have antagonists. So antagonists will block receptors in the body. So for example, we have naloxone, which is an opioid antagonist. Naloxone will block those opioid receptors, which reverses the analgesic effect of those opioid medications.

Occasionally, you may see a medication that is an agonist-antagonist - right? - so it's both. We sometimes refer to these as partial agonist. With agonist-antagonists, these medications act as an agonist or partial agonist at some receptors, and they will also act as an antagonist at other receptors. So if you see that, I wanted to make sure you knew what that meant.

All right. That is it for this video. Are you guys ready for your quiz? First question. What type of medication is safer? A medication with a high therapeutic index or a medication with a low therapeutic index? The answer is high therapeutic index. So if you remember, that means we have a lot of room between that minimum effective concentration and the concentration at which we worry about toxicity. Second question. Drugs with a longer half-life have a greater risk for toxicity. True or false? The answer is true. Last question. What type of medication blocks a receptor in the body? The answer is an antagonist. So hope you did well on that, and I'll see you soon with more videos. Take care.


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