Vomiting is coordinated by the vomiting center in the medulla, which receives input from the chemoreceptor trigger zone (CTZ) (dopamine D2, serotonin 5-HT3, neurokinin-1 receptors — senses drugs and toxins in blood), the GI tract (5-HT3 receptors on vagal nerves, released by chemotherapy and radiation), the vestibular system (histamine H1 and muscarinic receptors — motion sickness), and the cortex (anxiety, anticipation). Antiemetics are chosen by the receptor that drives the cause.

Laxatives increase stool water or bowel movement.

• Bulk-forming (psyllium, methylcellulose) — absorb water, enlarge stool; slowest, most physiologic (12–72 hours)
• Osmotic (polyethylene glycol [PEG], lactulose, magnesium hydroxide or citrate, sodium phosphate) — draw water into the bowel
• Stimulant (senna, bisacodyl) — stimulate the myenteric plexus and increase secretion (6–12 hours orally; suppository within an hour)
• Stool softener (docusate) — surfactant; weak evidence alone
• Lubricant (mineral oil)
• Secretagogues (lubiprostone, linaclotide, plecanatide) — increase intestinal fluid secretion
• Peripherally acting mu-opioid receptor antagonists (PAMORAs) (methylnaltrexone, naloxegol, naldemedine) — block opioid effects in the gut without reversing analgesia

Antidiarrheals

• Loperamide and diphenoxylate — opioid agonists acting on gut receptors → slow motility; loperamide barely enters the brain at normal doses
• Bismuth subsalicylate — antisecretory and antimicrobial
• Octreotide — somatostatin analog; reduces secretory diarrhea (carcinoid, VIPoma, high-output ostomy)
• Oral rehydration solution (ORS) — glucose–sodium cotransport restores fluid; the most important treatment of acute diarrhea

Ondansetron: headache, constipation, dizziness, QT prolongation (dose-related; the single IV dose is limited to 16 mg), serotonin syndrome with other serotonergic drugs.

Dopamine antagonists: extrapyramidal symptoms (acute dystonia, akathisia — especially in children and young adults), tardive dyskinesia (metoclopramide), sedation, orthostatic hypotension, neuroleptic malignant syndrome (rare), QT prolongation (droperidol).

Promethazine: sedation, respiratory depression, anticholinergic effects; injection causes severe chemical irritation — gangrene with extravasation or intra-arterial injection.

Antihistamines/anticholinergics (scopolamine, meclizine): dry mouth, drowsiness, blurred vision, urinary retention, constipation, confusion in older adults.

Dexamethasone: hyperglycemia, insomnia. Dronabinol: dizziness, euphoria, tachycardia.

Laxatives

• Bulk-forming: bloating; esophageal or bowel obstruction if taken with too little water
• Osmotic: cramping, gas, diarrhea, dehydration and electrolyte loss (hypokalemia, hypernatremia with lactulose); hypermagnesemia from magnesium salts in kidney disease; sodium phosphate — hyperphosphatemia and acute phosphate nephropathy
• Stimulant: cramping, diarrhea; long-term overuse can cause electrolyte loss
• Mineral oil: lipoid pneumonia if aspirated, reduced absorption of fat-soluble vitamins, anal leakage
• Secretagogues: diarrhea; lubiprostone — nausea, dyspnea
• PAMORAs: abdominal pain, diarrhea, GI perforation (rare), opioid withdrawal symptoms

Antidiarrheals

• Loperamide: constipation, abdominal cramps; high doses (misuse) → QT prolongation, torsades, syncope, cardiac arrest
• Diphenoxylate–atropine: dry mouth, drowsiness, tachycardia; respiratory depression in children
• Octreotide: gallstones, glucose changes (hypo- or hyperglycemia), bradycardia, injection-site pain

Antiemetics

• QT-prolonging drugs (ondansetron, droperidol, prochlorperazine) — avoid with congenital long QT, hypokalemia or hypomagnesemia, and other QT drugs (some antiarrhythmics, macrolides, fluoroquinolones, antipsychotics, methadone)
• Metoclopramide: contraindicated in GI obstruction, perforation, or bleeding, pheochromocytoma, seizure disorders, and with other drugs causing EPS; reduce dose in kidney impairment; avoid in Parkinson disease
• Promethazine: contraindicated under 2 years; caution with respiratory disease; the preferred injection route is deep IM; IV use (December 2023 label) requires dilution to no more than 1 mg/mL and slow infusion over 20–40 minutes through a large vein (preferably a central line), never through hand or wrist veins; subcutaneous and intra-arterial injection are contraindicated
• Anticholinergics: narrow-angle glaucoma, urinary retention, prostatic hyperplasia; high-risk in older adults (Beers Criteria)
• Aprepitant: CYP3A4 interactions; reduces hormonal contraceptive effectiveness (backup for 1 month) and warfarin effect (check INR)
• Additive CNS depression with opioids, alcohol, benzodiazepines

Laxatives

• Do not give any laxative with undiagnosed abdominal pain, nausea and vomiting, suspected appendicitis, or bowel obstruction, or with fecal impaction above the rectum (manual removal or enema first)
• Magnesium and phosphate products: avoid in kidney impairment, heart failure; sodium phosphate enemas are avoided in young children and in kidney disease or dehydration
• Bisacodyl enteric-coated tablets: do not crush or chew; do not take within 1 hour of milk or antacids (early dissolution → gastric irritation)
• Mineral oil: avoid in clients at risk of aspiration (dysphagia, bedbound, young children)
• PAMORAs: contraindicated in known or suspected mechanical GI obstruction
• Psyllium separate from other drugs (can reduce absorption)

Antidiarrheals

• Avoid loperamide and diphenoxylate with bloody diarrhea, high fever, or suspected C. difficile or invasive bacterial infection (toxic megacolon, prolonged infection)
• Loperamide: not for children under 2; avoid with QT risk factors
• Diphenoxylate–atropine: contraindicated under 6 years (label); caution with liver disease; MAOIs
• Bismuth subsalicylate: avoid in children and teens with viral illness (Reye syndrome), with aspirin allergy or anticoagulants

1. Assess the cause first — nausea from bowel obstruction, raised intracranial pressure, pregnancy, or drug toxicity needs different management; constipation or diarrhea needs a cause assessment before drugs
2. Fluid and electrolyte status: vital signs, orthostatic changes, mucous membranes, urine output, daily weight, potassium, sodium, magnesium (normal potassium 3.5–5.0 mEq/L (mmol/L)); correct low potassium and magnesium before QT-prolonging antiemetics
3. Aspiration safety for vomiting clients with reduced alertness: side-lying position, suction ready
4. ECG when giving QT-prolonging antiemetics to clients with risk factors
5. Time antiemetics before the trigger: 30–60 minutes before chemotherapy; scopolamine patch at least 4 hours before travel
6. Watch for dystonia after metoclopramide or prochlorperazine; treat with diphenhydramine or benztropine as ordered; give IV metoclopramide slowly (over 1–2 minutes for a 10 mg bolus, or diluted) to reduce akathisia
7. Promethazine injection: preferred deep IM; if IV is ordered, dilute and infuse over 20–40 minutes through a large vein (not the hand or wrist); stop at once for pain or burning at the site
8. Bowel regimen with every opioid order (stimulant with or without softener); do not rely on bulk-forming laxatives alone for opioid constipation
9. Record stool frequency, consistency, blood; C. difficile testing for new diarrhea after antibiotics, with contact precautions and soap-and-water hand hygiene

• Take bulk-forming laxatives with a full glass (240 mL) of water and drink fluids through the day
• Increase fiber (gradually), fluids, and activity; respond to the urge to defecate
• Avoid long-term routine stimulant laxative use unless advised (for example, with ongoing opioid therapy)
• Scopolamine patch: apply to clean, hairless skin behind the ear; wash hands after touching (pupil dilation and blurred vision if rubbed into the eye); remove before MRI
• Antiemetics and antihistamines cause drowsiness — no driving or alcohol
• Loperamide: do not exceed the label dose (8 mg/day over the counter unless a provider directs up to 16 mg/day); stop and seek care for fever, blood in stool, or diarrhea beyond 48 hours
• Oral rehydration solution is the key treatment of diarrhea in children and adults; sports drinks and sodas are not substitutes
• Report severe abdominal pain, abdominal distension, vomiting with no stool or gas passing (possible obstruction)

• Match antiemetic to cause: 5-HT3 for chemo/postop; anticholinergic/antihistamine for motion sickness
• Ondansetron: QT prolongation, constipation; correct K⁺ and Mg²⁺
• Metoclopramide: tardive dyskinesia (boxed), no longer than 12 weeks; not in GI obstruction or seizures
• Promethazine: never under 2 years; deep IM preferred; tissue injury with IV
• Dystonia → diphenhydramine or benztropine
• No laxatives with undiagnosed abdominal pain or obstruction
• Bulk-forming laxatives need a full glass of water
• Opioids → start a stimulant laxative; PAMORAs contraindicated in obstruction
• No loperamide with bloody diarrhea, fever, or C. difficile; high doses → cardiac toxicity
• ORS is first-line for dehydration from diarrhea
• Magnesium and phosphate laxatives: avoid in kidney disease