Gastrointestinal Pharmacology – BCSE Study Guide

Overview and Clinical Importance

Gastrointestinal (GI) disorders are among the most common presentations in veterinary practice across all species. Understanding GI pharmacology is essential for the entry-level veterinarian because these drugs are used daily for managing vomiting, diarrhea, constipation, gastric ulcers, and motility disorders. The BCSE tests your ability to select appropriate drugs based on mechanism of action, species differences, and clinical indications.

This guide covers five major categories of GI drugs: Antiemetics, Antidiarrheals, Laxatives and Cathartics, Antacids/H2 Blockers/Proton Pump Inhibitors, and Prokinetics. Each section emphasizes mechanisms of action, clinical applications, species considerations, and common exam pitfalls.

Section 1: Antiemetics

Antiemetics are drugs that prevent or treat vomiting (emesis). Understanding the emetic pathway is crucial for selecting the appropriate antiemetic. The vomiting reflex involves multiple receptors in the chemoreceptor trigger zone (CRTZ), emetic center, vestibular apparatus, and peripheral GI tract.

The Emetic Pathway and Receptor Targets

The chemoreceptor trigger zone (CRTZ) is located in the area postrema of the medulla and lies outside the blood-brain barrier, allowing it to detect circulating toxins, drugs, and metabolites. The emetic center receives input from the CRTZ, vestibular apparatus, and vagal afferents from the GI tract. Different receptors predominate in different species and anatomical locations:

MEMORY AID - CATS Use Different Receptors: Think "CATS Are Special" - Cats rely on Alpha-2 and Serotonin (5-HT3) receptors in the CRTZ, NOT dopamine. This is why metoclopramide is less effective as an antiemetic in cats!

[Include Image: Figure 1. Emetic pathway showing CRTZ, emetic center, and receptor locations]

NK-1 Receptor Antagonists

Maropitant (Cerenia)

Mechanism: Maropitant is a selective neurokinin-1 (NK-1) receptor antagonist that blocks the binding of Substance P in both the emetic center and CRTZ. This dual action provides broad-spectrum antiemetic activity against both central (humoral) and peripheral (neural) emetic stimuli.

Clinical Applications: Acute vomiting from any cause (GI disease, toxins, drugs), motion sickness, chemotherapy-induced nausea and vomiting, and perioperative nausea. Maropitant is FDA-approved for dogs and cats.

Dosage: Dogs: 1 mg/kg SC or 2 mg/kg PO q24h. For motion sickness, 8 mg/kg PO given 2 hours before travel. Cats: 1 mg/kg SC or PO q24h.

Pharmacokinetics: Half-life approximately 6 hours in dogs. Oral bioavailability is approximately 30%. Hepatically metabolized. Safe for use in animals with renal disease.

MEMORY AID - MAROPitant for Motion And Regular vOmiting Prevention: Maropitant blocks NK-1 receptors where Substance P acts - it works at the "control center" of vomiting (emetic center) so it stops vomiting from almost any cause!

Serotonin (5-HT3) Receptor Antagonists

Ondansetron (Zofran) and Dolasetron

Mechanism: Selective 5-HT3 receptor antagonists that block serotonin receptors in the CRTZ (area postrema) and on vagal nerve terminals in the GI tract. Chemotherapy and radiation damage GI mucosa, releasing serotonin that triggers vomiting via these receptors.

Clinical Applications: Highly effective for chemotherapy-induced nausea and vomiting. Also used for acute vomiting and as second-line antiemetics. NOT effective for motion sickness.

Dosage: Ondansetron: 0.1-0.2 mg/kg IV or PO q8-12h; for chemotherapy: 0.5 mg/kg IV. Dolasetron: 0.6 mg/kg IV or PO q24h.

MEMORY AID - SetRONS Stop Serotonin: OndanSETRON, dolaSETRON - drugs ending in "-setron" are 5-HT3 (Serotonin) antagonists. Think "SET" = Serotonin Emesis Treatment. Best for chemotherapy-induced vomiting!

Dopamine (D2) Receptor Antagonists

Metoclopramide (Reglan)

Mechanism: Metoclopramide has multiple mechanisms: (1) D2 receptor antagonism in the CRTZ (antiemetic), (2) 5-HT4 receptor agonism in the GI tract (prokinetic), and (3) At high doses, 5-HT3 receptor antagonism. It increases lower esophageal sphincter tone and accelerates gastric emptying.

Clinical Applications: Useful antiemetic in DOGS (less effective in cats). Also provides prokinetic effects for delayed gastric emptying and reflux esophagitis.

Dosage: Dogs and Cats: 0.2-0.5 mg/kg PO, SC, or IV q6-8h. Can be given as CRI at 1-2 mg/kg/day.

Adverse Effects: CNS effects due to dopamine antagonism: restlessness, agitation, behavioral changes, extrapyramidal signs. Contraindicated in epileptic patients (lowers seizure threshold) and with suspected GI obstruction (prokinetic effects).

MEMORY AID - Metoclopramide = Multiple Mechanisms in Mutts: Metoclopramide works on D2 (Dopamine), 5-HT4 (prokinetic), and 5-HT3 (at high doses). It is a "jack of all trades" but works best in DOGS ("Mutts") because dogs have more D2 receptors in their CRTZ.

Phenothiazines (Chlorpromazine, Acepromazine)

Phenothiazines are D2 antagonists with additional alpha-1 adrenergic and H1 antihistamine effects. Chlorpromazine (0.5 mg/kg SC q6-8h) is the most commonly used phenothiazine antiemetic. They cause sedation and hypotension, limiting their use. Less effective than maropitant or ondansetron for chemotherapy-induced vomiting.

Antiemetic Drug Comparison Table

Section 2: Antidiarrheals

Antidiarrheal drugs are used to manage acute and chronic diarrhea by reducing intestinal motility, enhancing fluid absorption, or providing mucosal protection. Understanding the underlying cause of diarrhea is essential because some antidiarrheals are contraindicated in infectious diarrhea where increased motility helps eliminate pathogens.

Opioid Antidiarrheals

Loperamide (Imodium)

Mechanism: Loperamide is a synthetic opioid that acts on mu-opioid receptors in the GI tract to: (1) Stimulate circular smooth muscle contraction, increasing intestinal segmentation (slowing transit), (2) Stimulate absorption and inhibit secretion of fluid and electrolytes, (3) Increase anal sphincter tone (may help fecal incontinence).

Clinical Applications: Acute non-infectious diarrhea in dogs. May be used for chronic large bowel diarrhea and IBD management. NOT recommended for cats due to potential excitement.

Dosage: Dogs: 0.08-0.2 mg/kg PO q8-12h. Use with caution.

MEMORY AID - COLLIES Cannot Cope with Loperamide: Herding breeds (Collies, Aussies, Shelties) lack the P-glycoprotein "border patrol" (MDR1 mutation) that keeps loperamide OUT of the brain. Without this "guard," loperamide causes severe sedation!

Contraindications: MDR1-mutant breeds, infectious diarrhea (parvovirus, Salmonella), suspected GI obstruction, hepatic encephalopathy, debilitated patients, Addison's disease, hypothyroidism, increased intracranial pressure. Do not use when retention of intestinal toxins would be harmful.

Diphenoxylate (Lomotil)

Diphenoxylate is another synthetic opioid antidiarrheal, typically combined with atropine (to discourage abuse). Dosage in dogs: 0.1-0.2 mg/kg PO q8-12h. Similar contraindications to loperamide. Also affected by MDR1 mutation.

Intestinal Protectants and Adsorbents

Bismuth Subsalicylate (Pepto-Bismol)

Mechanism: Coats and protects the intestinal mucosa, reduces inflammation via salicylate component, has mild antibacterial activity, and decreases intestinal secretions.

Dosage (Dogs only): 1 mL/kg (or approximately 1 teaspoon per 10 lbs body weight) PO q6-8h.

MEMORY AID - CATS Cannot Cope with saliCylATes: Bismuth subsaliCYLATE = saliCYLATE = toxic to CATS. If you see "salicylate" in the name, think "no cats!"

Kaolin-Pectin

Kaolin (clay) and pectin (plant fiber) provide intestinal protection by adsorbing toxins and bacteria and coating the intestinal mucosa. Generally safe for both dogs and cats with minimal contraindications. Dosage: 1-2 mL/kg PO q4-6h. Less effective than opioid antidiarrheals but safer for mild cases.

Antimicrobials for GI Disease

Metronidazole (Flagyl)

Mechanism: Metronidazole is a nitroimidazole antibiotic with anaerobic antibacterial activity AND immunomodulatory/anti-inflammatory effects. It inhibits cell-mediated immunity and is effective against Giardia and other protozoa.

Clinical Applications: Inflammatory bowel disease (IBD), Giardia infection, Clostridium perfringens enterotoxicosis, small intestinal bacterial overgrowth (SIBO), and hepatic encephalopathy.

Dosage: Dogs and Cats: 10-15 mg/kg PO q12h (for IBD), 25 mg/kg PO q12h for 5-7 days (for Giardia).

Adverse Effects: Neurotoxicity at high doses or prolonged use (ataxia, nystagmus, head tilt, seizures). Anorexia and vomiting (bitter taste). Avoid in pregnancy (teratogenic).

Sulfasalazine

Mechanism: Sulfasalazine is a compound of 5-aminosalicylate (5-ASA, mesalamine) linked to sulfapyridine. Colonic bacteria cleave the molecule, releasing 5-ASA locally in the colon where it inhibits prostaglandin synthesis and provides anti-inflammatory effects.

Clinical Applications: Large bowel inflammatory bowel disease (colitis) in dogs. Use with caution in cats due to salicylate sensitivity.

Dosage: Dogs: 10-25 mg/kg PO q8h for 4-6 weeks, then taper. Cats (if used): 5-12.5 mg/kg PO q8h.

MEMORY AID - SulfaSALazine = SAL = Salicylate for Large bowel: Sulfasalazine releases 5-ASA (SALicylate) in the LARGE bowel (colon). Use for COLITIS. Watch for dry eyes (KCS)!

[Include Image: Figure 2. Sulfasalazine structure showing azo bond cleavage by colonic bacteria]

Section 3: Laxatives and Cathartics

Laxatives and cathartics are used to treat constipation by various mechanisms: increasing stool bulk, softening feces, drawing water into the intestine, or stimulating intestinal motility. Cathartics generally produce stronger and faster effects than laxatives. Chronic constipation in cats (often progressing to megacolon) is a common clinical problem requiring lifelong management.

Classification of Laxatives

MEMORY AID - BOLES - Types of Laxatives: B = Bulk-forming (fiber), O = Osmotic (water-drawing), L = Lubricant (oils), E = Emollient (stool softeners), S = Stimulant (nerve stimulation). Remember "BOLES" moves bowels!

Osmotic Laxatives (Most Important for BCSE)

Lactulose

Mechanism: Lactulose is a synthetic disaccharide that is NOT absorbed. It reaches the colon where bacteria ferment it into organic acids (lactic acid, acetic acid). These acids: (1) Create osmotic pressure, drawing water into the bowel, (2) Stimulate colonic motility, (3) Lower colonic pH, converting ammonia (NH3) to ammonium ions (NH4+) which cannot be absorbed, thereby reducing systemic ammonia levels.

Clinical Applications: Chronic constipation (especially in cats with megacolon), Hepatic encephalopathy (primary use - reduces ammonia absorption).

Dosage: Dogs and Cats: 0.5 mL/kg PO q8-12h for constipation. Titrate to achieve soft stools. For hepatic encephalopathy: higher doses (1 mL/kg q6-8h) or given as enema.

Adverse Effects: Flatulence, bloating, diarrhea (if overdosed), cramping. Some cats dislike the sweet taste.

MEMORY AID - Lactulose Lowers (ammonia) and Loosens (stool): LACTulose: LACT = Lowers Ammonia in Colon (for HE) and also Loosens ACTivates bowels (laxative). Two L-functions!

Polyethylene Glycol 3350 (PEG, Miralax)

Mechanism: PEG 3350 is a large, water-soluble polymer that is NOT metabolized or absorbed. Each molecule forms hydrogen bonds with approximately 100 water molecules, creating high osmotic pressure within the bowel lumen that prevents water absorption and softens stool.

Clinical Applications: Chronic constipation in cats and dogs. Well-tolerated for long-term use. Unlike lactulose, does NOT cause bloating or flatulence because it is not fermented.

Dosage: Cats: 1/8 to 1/4 teaspoon mixed with food q12-24h. Dogs: 1/8 to 1/4 teaspoon per 10 lbs body weight q12-24h. Titrate to effect.

MEMORY AID - PEG = Pulls Electrolytes into Gut (via water): PEG-3350 is safe - the "3350" is just the molecular weight. It PULLS water into the gut osmotically. No fermentation = no gas!

Important Warnings for Laxatives

NEVER give sodium phosphate enemas (Fleet enemas) to CATS! Cats rapidly absorb phosphate from these enemas, leading to fatal hyperphosphatemia, hypocalcemia, hypernatremia, and hyperosmolality. Use warm water or saline enemas instead.

Do not combine mineral oil with docusate (DSS). Docusate increases absorption of mineral oil, potentially causing lipid pneumonia if aspirated or hepatotoxicity if absorbed systemically.

Section 4: Gastric Acid Suppressants

Gastric acid suppressants are used to treat gastric and duodenal ulcers, gastroesophageal reflux disease (GERD), and stress-related mucosal disease. Understanding the mechanisms of gastric acid secretion is essential for selecting appropriate therapy.

Gastric Acid Secretion Physiology

Gastric parietal cells secrete hydrochloric acid (HCl) via the H+/K+-ATPase proton pump on their apical membrane. Three main stimulatory pathways converge on parietal cells: (1) Histamine from enterochromaffin-like (ECL) cells acts on H2 receptors, (2) Acetylcholine from vagal neurons acts on M3 muscarinic receptors, (3) Gastrin from G cells acts on gastrin/CCK-B receptors. All three pathways ultimately stimulate the proton pump to secrete acid.

[Include Image: Figure 3. Parietal cell diagram showing H2, M3, and gastrin receptors converging on H+/K+-ATPase pump]

Antacids

Mechanism: Antacids are weak bases (aluminum hydroxide, magnesium hydroxide, calcium carbonate) that neutralize gastric acid already present in the stomach. They do NOT reduce acid secretion. They also decrease pepsin activity and may stimulate local prostaglandin production.

Clinical Limitations: Antacids have SHORT duration of action (30-60 minutes) and require frequent dosing. They are inferior to H2 blockers and PPIs for sustained acid suppression. Not commonly used as primary therapy in veterinary medicine.

Histamine H2 Receptor Antagonists (H2RAs)

Famotidine (Pepcid) and Ranitidine

Mechanism: H2RAs competitively block histamine H2 receptors on parietal cells, reducing acid secretion stimulated by histamine. They block only ONE of the three pathways to acid secretion, so their acid suppression is incomplete.

Potency: Famotidine is 20-150 times more potent than cimetidine. Ranitidine is 3-13 times more potent than cimetidine. Famotidine is the most commonly used H2RA in veterinary medicine.

Dosage: Famotidine: Dogs and Cats: 0.5-1 mg/kg PO, IV, or SC q12-24h.

Characteristics: Rapid onset of action (within 1 hour). Can be given with food. Develops TACHYPHYLAXIS (tolerance) with prolonged use - efficacy decreases after 3+ days of continuous use.

MEMORY AID - H2 Blockers are "2nd Best": H2 blockers block only ONE pathway (histamine). PPIs block the FINAL common pathway (proton pump). Think "H2 = 2nd choice" for long-term acid suppression!

Proton Pump Inhibitors (PPIs)

Omeprazole (Prilosec, GastroGard)

Mechanism: PPIs IRREVERSIBLY bind to and inactivate the H+/K+-ATPase proton pump on parietal cells. This blocks the FINAL COMMON PATHWAY of acid secretion, regardless of the stimulus (histamine, acetylcholine, or gastrin). PPIs are prodrugs that become activated in the acidic environment of the parietal cell canaliculus.

Dosage: Dogs: 1-2 mg/kg PO q12h (twice daily is needed for optimal acid suppression). Cats: 1 mg/kg PO q12-24h. Horses: 4 mg/kg PO q24h (GastroGard).

Clinical Pearls: Give on an empty stomach, 30 minutes BEFORE the first meal of the day. PPIs are most effective when parietal cells are actively secreting acid (preparing for a meal). Takes 2-5 days to reach maximum effect as more proton pumps are inhibited.

MEMORY AID - OMEprazole = O My, the Most Effective: PPIs ending in "-prazole" (OMEprazole, pantoPRAZOLE, esomePRAZOLE) are the MOST effective acid suppressants. They block the "PUMP" - the final common pathway!

Gastric Acid Suppressant Comparison

Section 5: Prokinetics

Prokinetic drugs enhance gastrointestinal motility by stimulating coordinated contractions. They are used to treat delayed gastric emptying, gastroesophageal reflux, ileus, and chronic constipation (especially megacolon in cats).

Serotonergic Prokinetics

Cisapride

Mechanism: Cisapride is a 5-HT4 receptor agonist that enhances acetylcholine release from myenteric plexus neurons, increasing coordinated peristalsis throughout the GI tract from esophagus to colon. It has some 5-HT3 antagonist activity as well.

Clinical Applications: MOST EFFECTIVE prokinetic for FELINE MEGACOLON. Also used for delayed gastric emptying, postoperative ileus, and gastroesophageal reflux. Unlike metoclopramide, cisapride has activity throughout the entire GI tract including the colon.

Dosage: Dogs and Cats: 0.1-0.5 mg/kg PO q8-12h, given 30 minutes before meals. Higher doses (1 mg/kg) may be needed in refractory cases.

Availability: Cisapride was removed from the US human market in 2000 due to fatal cardiac arrhythmias (QT prolongation, torsades de pointes) in humans. It is available through veterinary compounding pharmacies. Cardiac effects have NOT been reported clinically in dogs and cats.

MEMORY AID - CISapride = Colon In Service: CISapride works on the COLON (unlike metoclopramide). Think "CIS" = "Colon In Service" for megacolon cats! Give 30 minutes before meals.

Dopaminergic Prokinetics

Metoclopramide (Reglan)

Prokinetic Mechanism: Metoclopramide acts as a 5-HT4 agonist and D2 antagonist to increase acetylcholine release in the proximal GI tract. It increases lower esophageal sphincter tone, gastric contractions, and antropyloroduodenal coordination. However, it has MINIMAL effects on the colon.

Clinical Applications: Delayed gastric emptying, gastroesophageal reflux, and ileus affecting the proximal GI tract. More commonly used for its antiemetic effect combined with mild prokinetic benefit.

Limitations: NOT effective for colonic motility disorders (megacolon). CNS side effects (extrapyramidal signs) due to dopamine antagonism. Contraindicated in patients with GI obstruction, epilepsy, or pheochromocytoma.

MEMORY AID - Metoclopramide = Mouth to Middle (not colon): Metoclopramide works from the MOUTH (esophagus) to the MIDDLE (stomach, small intestine) but NOT the colon! For megacolon, you need CISapride.

Motilinergic Prokinetics

Erythromycin

Mechanism: Erythromycin (at LOW, sub-antimicrobial doses) acts as a motilin receptor agonist, stimulating phase III migrating motor complex (MMC) activity in the stomach and small intestine. In dogs, it works via motilin receptors, increased endogenous motilin release, and 5-HT3 cholinergic pathways.

Clinical Applications: Delayed gastric emptying, postoperative ileus. Can accelerate gastric emptying but may result in inadequately digested food entering the small intestine. Dogs: increases colonic activity. Cats: direct smooth muscle motilin receptor agonist (different mechanism).

Dosage: Dogs: 0.5-1 mg/kg IV or PO q8h (prokinetic dose - much lower than antimicrobial dose). May develop tachyphylaxis with chronic use.

MEMORY AID - Erythromycin = Eating Rush (at low doses): ERYTHROmycin at low doses makes food "RUSH" through the stomach by mimicking motilin. Remember: LOW dose = prokinetic; HIGH dose = antibiotic!

Prokinetic Drug Comparison