Pharmacology, part 6: “Therapeutic Index, Half-Life, Mode of Action”
by Cathy Parkes January 16, 2023 Updated: August 11, 2023 3 min read
Hi, I'm Cathy with Level Up RN. In this video, I am going to continue my coverage of pharmacology basics. Specifically, I will be talking about therapeutic index, half-life, and the mode of action of medications. At the end of the video, I'm going to give you guys a little quiz to test your knowledge of some of the key points I'll be covering in this video. So definitely stay tuned for that. And I will be following along with our Level Up RN pharmacology flashcards, so if you have our flashcards, definitely pull them out so you can follow along with me. And pay close attention to the bold red text on the cards because these represent the most important facts that you are likely to get tested on.
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-antagonist, 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 want to make sure you knew what that meant.
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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