Antiemetics
Key Points
- Multiple drug classes treat nausea/vomiting by targeting different neurotransmitter receptors in the chemoreceptor trigger zone (CTZ) and vomiting center
- Ondansetron (Zofran): Serotonin antagonist — first-line for chemotherapy-induced nausea/vomiting (CINV) and postoperative nausea
- Prochlorperazine: Dopamine antagonist — effective but causes extrapyramidal symptoms (EPS) with prolonged use
- Metoclopramide: Prokinetic — speeds gastric emptying; risk of tardive dyskinesia with long-term use (Boxed Warning)
- Meclizine: Antihistamine — first-line for motion sickness and vertigo
- Scopolamine: Anticholinergic transdermal patch — motion sickness prevention; remove before MRI (metallic aluminized membrane)
Pathophysiology of Nausea and Vomiting
The chemoreceptor trigger zone (CTZ), located in the area postrema of the brain, receives input from multiple pathways and is not restricted by the blood-brain barrier — allowing it to detect toxins in the bloodstream (e.g., opioids, chemotherapy agents, anesthesia).
CTZ receives input from:
- GI tract (via cranial nerves IX and X) — distension, obstruction, inflammation
- Vestibular system — motion sickness, vertigo
- Cerebral cortex — odors, anxiety, pain
- Bloodstream — drugs, toxins, metabolic disturbances
The CTZ activates the vomiting center in the medulla → retro-peristaltic contractions + abdominal muscle contraction → emesis.
Drug Classes and Mechanisms
| Neurotransmitter | Drug Class | Prototype | Mechanism |
|---|---|---|---|
| Serotonin (5-HT3) | Serotonin antagonists | Ondansetron | Blocks 5-HT3 in GI tract, CTZ, and vomiting center |
| Dopamine (D2) | Dopamine antagonists | Prochlorperazine | Blocks dopamine in CTZ |
| Dopamine + ACh | Prokinetics | Metoclopramide | Blocks dopamine in CTZ + stimulates ACh in GI tract |
| Histamine (H1) | Antihistamines | Meclizine | Blocks H1 receptors → blocks ACh in vestibular system |
| Acetylcholine (M1) | Anticholinergics | Scopolamine | Blocks ACh receptors in vestibular system |
| Substance P (NK1) | Neurokinin antagonists | Aprepitant | Blocks NK1 receptors — used for CINV |
| Cannabinoid (CB1) | THC | Dronabinol | Activates CB1 receptor — inhibitory effect on CTZ |
Serotonin Antagonists (5-HT3 Antagonists)
Ondansetron (Zofran): Most widely used antiemetic in clinical practice.
Indications: Chemotherapy-induced nausea and vomiting (CINV), postoperative nausea and vomiting (PONV), radiation-induced nausea.
Advantages: Excellent safety profile; does not cause sedation or EPS; safe in pregnancy (used for hyperemesis gravidarum).
Routes: Oral, ODT (orally disintegrating tablet), IV, IM. Common pregnancy-context dosing reference: 4-8 mg every 8 hours as prescribed. IV timing profile: Onset often within about 15 minutes, peak around 30 minutes, and duration about 4-8 hours. IV administration safety: Give by slow IV push per protocol (commonly over about 2-5 minutes in adult practice) and reassess IV-site patency during/after administration to reduce infiltration or extravasation risk.
Adverse Effects: Headache, constipation, QTc prolongation with IV use (monitor ECG in at-risk patients).
Dopamine Antagonists
Prochlorperazine (Compazine): Blocks dopamine D2 receptors in the CTZ.
Indications: Severe nausea and vomiting, migraine-associated nausea.
Adverse Effects:
- Extrapyramidal Symptoms (EPS): Akathisia, dystonia, drug-induced parkinsonism — especially with prolonged use
- Sedation, orthostatic hypotension
- Tardive dyskinesia (long-term use)
Prokinetics — Metoclopramide (Reglan)
Mechanism: Blocks dopamine D2 receptors in the CTZ AND stimulates acetylcholine receptors in the GI tract → accelerates gastric emptying.
Indications: Diabetic gastroparesis, GERD, PONV, N/V when gastric emptying delay is a component.
Metoclopramide Boxed Warning — Tardive Dyskinesia
Tardive dyskinesia (irreversible involuntary movements) has been reported with metoclopramide. Risk increases with duration of use and cumulative dose. Avoid use >12 weeks unless clinical benefit outweighs risk.
Adverse Effects: EPS, drowsiness, fatigue, restlessness (akathisia).
Antihistamines
Meclizine (Antivert): Blocks H1 receptors, secondarily blocking acetylcholine in the vestibular system.
Indications: Motion sickness (first-line), vertigo associated with vestibular disorders.
Adverse Effects: Drowsiness, dry mouth, blurred vision — classic anticholinergic effects.
Pyridoxine-Doxylamine Combination (Pregnancy Nausea)
This combination is commonly used as first-line pharmacologic treatment when nonpharmacologic nausea measures are insufficient in pregnancy.
- Typical administration pattern: Often started as bedtime dosing, with additional daytime doses per prescription if symptoms persist.
- Mechanism pair: Pyridoxine (vitamin B6 support) plus doxylamine (H1-antihistamine/anticholinergic anti-nausea effect).
- Important caution: Avoid concurrent MAOI therapy due to interaction risk.
- Common side effects: Drowsiness, dry mouth/throat, dizziness.
Anticholinergics
Scopolamine (Transderm Scōp): Transdermal patch releasing ~1 mg over 3 days.
Application: Apply to hairless skin behind the ear (postauricular area).
Indications: Prevention of motion sickness, postoperative nausea (associated with anesthesia and opioids).
MRI Contraindication
Scopolamine transdermal patches contain an aluminized membrane — remove before MRI to prevent skin burns at application site.
Adverse Effects: Dry mouth, blurred vision, drowsiness, confusion and memory impairment (especially in older adults), urinary retention.
Contraindication: Narrow-angle glaucoma.
Nursing Assessment
Before Administration:
- Identify the cause of nausea/vomiting — target treatment to the cause
- Assess hydration status: skin turgor, mucous membranes, urine output and color, blood pressure, heart rate
- Lab values if ordered: Hgb, Hct, serum sodium (dehydration markers)
- Document frequency, amount, and character of emesis
During/After Administration:
- Monitor for improvement in nausea and frequency of vomiting
- Assess for adverse effects (EPS with prochlorperazine/metoclopramide)
- Monitor continued fluid balance
- Report persistent vomiting >24 hours, hematemesis, or severe abdominal pain
Patient Education — Nonpharmacological Measures:
- Drink small sips of clear liquids frequently — avoid large amounts
- Eat bland foods; avoid spicy, fatty, or salty foods
- Eat smaller, more frequent meals
- Avoid strong smells that trigger nausea
- Prefer room-temperature meals/fluids and avoid high-bulk meals when nausea is active
- Use adjunct relaxation and music-therapy strategies to reduce symptom intensity
- Consider peppermint-oil aromatherapy as an adjunct when compatible with facility policy and patient preference
- For morning sickness: eat plain crackers before rising
Related Concepts
- diarrhea-assessment-and-management — GI distress management
- constipation — Opioid-induced constipation
- fluid-volume-deficit-hypovolemia-and-dehydration — Dehydration from persistent vomiting
- pain-management — Opioid-induced nausea management
- antidepressants — Ondansetron used with serotonergic drugs — serotonin syndrome risk
- postoperative-pacu-priorities-and-complication-surveillance — PONV as common postoperative complication
Self-Check
- A patient undergoing chemotherapy develops severe nausea. Which antiemetic class (and specific drug) is first-line for chemotherapy-induced nausea, and what is its mechanism?
- A patient who has been taking metoclopramide for 6 months develops repetitive lip-smacking movements and tongue thrusting. What adverse effect has occurred, and what is its significance?
- A patient with a scopolamine patch is scheduled for an MRI. What action must the nurse take before the scan, and why?