Pharmacology, part 38: Gastrointestinal Medications - Antidiarrheals and Antiemetics
by Cathy Parkes September 10, 2021 Updated: December 29, 2021 7 min read
In this article, we continue our coverage of gastrointestinal medications, specifically, antidiarrheal and antiemetic medications.
The Nursing Pharmacology video series follows along with our Pharmacology Flashcards, which are intended to help RN and PN nursing students study for nursing school exams, including the ATI, HESI, and NCLEX.
When you see this Cool Chicken, that indicates one of Cathy's silly mnemonics to help you remember. The Cool Chicken hints in these articles are just a taste of what's available across our Level Up RN Flashcards for nursing students!
Antidiarrheals are drugs that relieve symptoms of diarrhea, the frequent passage of a watery loose stool. They include loperamide (Imodium) and diphenoxylate/atropine (Lomotil).
Loperamide is a synthetic drug of the opiate class which inhibits peristalsis and is used to treat diarrhea.
Mode of action
Loperamide works by inhibiting peristalsis and prolonging the transit time of fecal matter in the intestines. It reduces fecal volume, increases fecal bulk and viscosity, and decreases the loss of electrolytes.
Among the side effects of loperamide are constipation, drowsiness, and dry mouth. Loperamide is a synthetic drug of the opiate class, which means it can be addictive. When used with much higher than the recommended doses, serious heart problems and death can occur. For this reason, the Food and Drug Administration (FDA) has limited the amount of the drug available in over-the-counter sales. When used as recommended, loperamide is safe.
Diphenoxylate/atropine is an antidiarrheal agent that is a combination drug of these two medications.
Mode of action
Diphenoxylate is a man-made narcotic chemically related to meperidine (Demerol). Diphenoxylate/atropine works by decreasing GI motility (the process of food traveling through the digestive tract).
The side effects of diphenoxylate/atropine include anticholinergic side effects, that is, this is a medication that results in drying (the patient can’t urinate or spit, for example). These effects are due to the drug’s atropine component.
In addition to the anticholinergic side effects of constipation, urinary retention, and dry mouth, other side effects associated with this medication can include dizziness, as well as the risk for dependence. That's because diphenoxylate is related chemically to a narcotic and can induce a feeling of euphoria in the patient. This drug is combined with atropine, because taking too much atropine will cause those unpleasant anticholinergic side effects.
Both loperamide and diphenoxylate/atropine can cause dry mouth, the easiest remedy for which is to suck on a hard candy or to chew gum.
I threw up on Dan. Better get me some ondansetron.
Antiemetics are drugs used to treat or prevent nausea and vomiting. These include ondansetron and promethazine and meclizine.
Ondansetron, used alone or with other medications, is often prescribed to patients undergoing cancer drug treatment (chemotherapy) and radiation therapy. Ondansetron and its related medications are used to treat nausea and vomiting.
Mode of action
These medications work by blocking serotonin receptors in the CTZ, the chemoreceptor trigger zone, an area of the medulla oblongata that receives inputs from blood-borne drugs or hormones, and communicates with other structures in the vomiting center to initiate vomiting.
The side effects of ondansetron include headache, constipation, or diarrhea. It also can cause QT prolongation, an irregularity of the electrical activity of the heart that places patients at risk for ventricular arrhythmias. This requires monitoring of the patient’s EKG.
Promethazine (Phenergan), meclizine (Antivert)
Two other medications that are used to treat nausea and vomiting include promethazine and meclizine. Not only can promethazine and meclizine be used for nausea and vomiting, they can also be used to treat motion sickness, gastroparesis (the partial paralysis of the stomach, where the stomach cannot empty itself of food normally) and GERD (gastroesophageal reflux disease).
Mode of action
These medications work by blocking the effects of histamine, and are often used to treat allergies. They also work by blocking anticholinergic activity.
The side effects of promethazine and meclizine can include drowsiness and dry mouth.
Emetics for patients undergoing chemotherapy
Note that if administering an antiemetic to a patient who is receiving chemotherapy — which can cause nausea and vomiting — it is important to administer the antiemetic prior to therapy rather than using it to treat nausea and vomiting that is already occurring in order to try to control this situation from the outset.
Prokinetic agents: metoclopramide (Reglan)
Metoclopramide is a prokinetic agent. In terms of the GI tract, prokinetics refer to a class of drugs that promote gastrointestinal motility (that is, transit). Metoclopramide can be used to treat nausea and vomiting, as well as gastroparesis. Gastroparesis is typically caused by diabetes, and it results in delayed gastric emptying. Metoclopramide can also be used to treat GERD.
Mode of action
Metoclopramide works by accelerating gastric emptying. In terms of nausea and vomiting, the medication attempts to get the stomach contents down into the intestines before the patient has a chance to throw it up. It also blocks dopamine and serotonin receptors in the CTZ (chemoreceptor trigger zone).
The side effects of metoclopramide can include drowsiness and GI upset. It can also cause extrapyramidal symptoms. These symptoms may include muscle rigidity, tremors, twitching, restlessness, and tardive dyskinesia (TD), a disorder that results in involuntary, repetitive body movements, which may include grimacing, sticking out the tongue, or smacking the lips.
Metoclopramide carries a black box warning because of the side effect of tardive dyskinesia. In terms of patient teaching, have the patient take this medication 30 minutes before meals and at bedtime.
Hi, I'm Cathy with Level Up RN. In this video, I'm going to continue our coverage of gastrointestinal medications, and specifically, I'll be covering antidiarrheal and antiemetic medications. At the end of the video, I'm going to provide you guys a quiz to test your knowledge of some of the key points I'll be covering in this video. So definitely stay tuned for that. If you have our Pharmacology 2nd Edition Flashcards, definitely pull those out, so you can follow along as well.
Let's start with our antidiarrheal. So Loperamide is one antidiarrheal that I would know. It works by inhibiting peristalsis and increasing the transit time of fecal matter in the intestines. Side effects can include constipation, drowsiness, and dry mouth.
Another antidiarrheal agent that's going to be important to know is Diphenoxylate/Atropine. So it's a combination drug of these two medications. So this medication works by decreasing GI motility. Side effects include anticholinergic side effects because of that Atropine component of this medication. So anticholinergic side effects can include constipation, urinary retention, and dry mouth. Other side effects can include dizziness, as well as the risk for dependence because the first part of that medication, Diphenoxylate, is actually, chemically related to a narcotic, and it can cause a feeling of euphoria. So they combined it with Atropine such that, if you take too much of this medication, you're going to get those yucky anticholinergic side effects. So it really discourages abuse of this medication. So with either of these antidiarrheal agents, so Loperamide or Diphenoxylate/Atropine, dry mouth is definitely a side effect. So you want to encourage your patient to suck on hard candy or chew gum to help alleviate that side effect.
Next, we have our antiemetics. So these are medications that are used to treat nausea and vomiting, and I know all about these medications because I had hyperemesis when I was pregnant. So they tried all of these things pretty much to no avail. But I do know about them. So let's go over the key meds I think you need to know.
The first one is Ondansetron. So this is used to treat nausea and vomiting, and it works by blocking that serotonin receptors in the CTZ, which is the chemoreceptor trigger zone. Side effects can include headache, constipation or diarrhea, as well as QT prolongation. So because of that side effect of QT prolongation, we're going to want to monitor our patients EKG to check for that.
A couple of other medications that are used to treat nausea and vomiting include Promethazine and Meclizine. So these medications can not only be used for nausea and vomiting, they can also be used for motion sickness. They work by blocking the effects of histamine, and they also have anticholinergic activity. Side effects can include drowsiness and dry mouth. So if we are administering an antiemetic for a patient who is receiving chemotherapy, which can obviously cause nausea and vomiting, we want to administer the antiemetic prior to therapy versus treating nausea and vomiting that is already occurring. So we want to get ahead of the situation.
Our little cool chicken hint for remembering Ondansetron is that I threw up on Dan and better get me some Ondansetron. So there's anyone named Dan who is watching this video, you should leave me a comment. Let me know how you feel about my hint, or if you've ever thrown up on somebody named Dan, I'd love to hear about that, too. So definitely leave me a comment.
Next, let's talk about the prokinetic agent, Metoclopramide. So Metoclopramide can be used to treat nausea and vomiting, as well as a disorder called gastroparesis. So gastroparesis is typically caused by diabetes, and it results in delayed gastric emptying. Metoclopramide can also be used to treat GERD. It works by accelerating gastric emptying. So in the case of nausea and vomiting, it's attempting to get the stomach contents down into the intestines before you have a chance to throw it up. Side effects of Metoclopramide can include drowsiness, GI upset, as well as extrapyramidal symptoms. So these extrapyramidal symptoms may include muscle rigidity, tremors, and tardive dyskinesia. So tardive dyskinesia is where the patient has these involuntary muscle movements. So Metoclopramide actually carries a black box warning because of the side effect of tardive dyskinesia. In terms of patient teaching, you want them to take this medication 30 minutes before meals and at bed time.
All right, you guys ready for a quiz? I have three questions for you. First question; what antidiarrheal agent can cause anticholinergic side effects and result independence? The answer is Diphenoxylate/Atropine. Second question; what antiemetic carries the risk for QT prolongation? The answer is Ondansetron. Third question; what antiemetic is used to treat gastroparesis by accelerating gastric emptying. The answer is Metoclopramide.
Hope you did well on that quiz. If you like my quiz questions, and this video was helpful for you, be sure to leave me a comment and like this video as well. Take care and good luck studying.
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