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.
Pharmacology - Nursing Flashcards
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.
1 comment
Your definition of Therapeutic Index (TI) is not the way I teach it. According to Goodman & Gilman (14th edition),
𝐓𝐈 = 𝑳𝑫𝟓𝟎/𝑬𝑫𝟓𝟎