What it is:Nausea is the subjective feeling that vomiting is coming. Vomiting is the actual forceful expulsion of gastric contents. They travel together through three phases: nausea (gastric stasis) → retching (labored abdominal/thoracic contractions) → vomiting (retroperistalsis expels contents).
The core problem:N/V isn't a disease, it's a final common output triggered by four completely different input pathways (toxins in the blood, inner-ear motion signals, gut irritation, and the brain itself). Whichever pathway is firing tells you which receptor to block.
What you do about it:Match the antiemetic's receptor target to the trigger. Motion sickness needs an anticholinergic/antihistamine, not a 5-HT3 blocker. Chemo needs 5-HT3 + NK-1 + steroid, not meclizine. Picking the wrong class is the single most common student error.
Think of the vomiting center in the medulla as a switchboard, not a trigger itself. It doesn't initiate anything on its own; it just integrates signals arriving from four separate lines (chemoreceptor trigger zone, vestibular apparatus, GI vagal afferents, and cortex/limbic system) and fires the efferent output once enough signal comes in. Every antiemetic class works by cutting one of those input lines, not by "calming the stomach."
N/V doesn't have a staging system like heart failure. Instead, you classify by cause, because cause dictates drug choice almost entirely. Six buckets cover nearly every exam question and every patient you'll see:
| Bucket | Classic trigger | First-line class |
|---|---|---|
| Motion sickness | Vestibular conflict (car, boat, plane) | Scopolamine patch (anticholinergic) |
| Nausea/vomiting of pregnancy (NVP) | Hormonal, first trimester | Pyridoxine ± doxylamine |
| Postoperative (PONV) | Anesthesia, surgery type, opioids | Risk-stratified combination (5-HT3 + dexamethasone ± others) |
| Chemotherapy-induced (CINV) | Emetogenic potential of the regimen | 5-HT3 + NK-1 + dexamethasone ± olanzapine |
| GI-origin(gastroenteritis, gastroparesis) | Mucosal irritation or delayed emptying | Ondansetron (gastroenteritis) or metoclopramide (gastroparesis, it's prokinetic) |
| Drug-induced | Opioids, anticonvulsants, digoxin, oral hypoglycemics, OCPs, volatile anesthetics | Remove/reduce offender first; treat symptomatically |
Beyond these, the handbook lists a long differential you should recognize but not memorize word for word: intraperitonealcauses (mechanical obstruction, gastroparesis, GERD, IBS, cyclic vomiting syndrome, cannabinoid hyperemesis syndrome), inflammatorycauses (pancreatitis, cholecystitis, appendicitis, hepatitis, gastroenteritis), cardiovascular(MI, cardiomyopathy), neurologic(increased ICP, migraine, vestibular disease, intracerebral hemorrhage or malignancy), metabolic(DKA, Addison's disease, uremia), and psychiatric(depression, anxiety, anorexia/bulimia).
Cannabinoid hyperemesis syndromeis a distractor students miss: chronic heavy cannabis use causes cyclic, severe vomiting that classically improves with hot showers/baths. It's the opposite of what you'd expect from an antiemetic drug class (dronabinol/nabilone), and standard antiemetics often fail; the fix is cannabis cessation.
Everything converges on the vomiting center, a diffuse nucleus of cells in the medulla. It doesn't sense anything itself. It just receives afferent signal from four places and, once threshold is crossed, sends efferent output to the salivation center, respiratory center, and the pharyngeal/GI/abdominal muscles, producing retching then vomiting.
Area postrema, sits outside the blood-brain barrier so it directly samples blood and CSF for toxins, drugs, and metabolic byproducts.
Inner ear motion/position sensors feed the vestibular nuclei. Fires with motion sickness, vertigo, and some migraines.
Mucosal injury (chemo, radiation, infection) triggers enterochromaffin cells to dump serotonin, which fires vagal afferents. Distension and gastroparesis also signal here.
Sight, smell, taste, pain, anxiety, and learned/anticipatory responses (classic in repeat chemo patients).
Read the four lines and the drug list writes itself. D2 and 5-HT3 and NK-1 antagonistshit the CTZ, which is why they dominate CINV and PONV (both are blood-borne/systemic insults). H1/M1 antagonistshit the vestibular line, which is why they're the answer for motion sickness and nothing else works nearly as well. 5-HT3 antagonistsalso cover the gut-afferent line because that's literally the receptor enterochromaffin cells use to signal. This is why combination regimens for CINV/PONV stack agents from differentlines rather than doubling up on one, since each drug only closes one gate.
A 5-HT3 antagonist plus dexamethasone plus an NK-1 antagonist covers three separate mechanisms, not three doses of the same mechanism. That's why triple therapy in high-risk CINV outperforms any single agent even at a higher dose. Dexamethasone's antiemetic mechanism is still not fully defined but likely involves blunting prostaglandin synthesis and central serotonin release, plus reducing peritumoral edema in CNS causes.
Simple N/Vis self-limiting, resolves without escalation, and responds to symptomatic therapy alone. No labs needed. Complex N/Vdoesn't respond to standard antiemetics, progressively worsens with fluid and electrolyte derangement, and is usually tied to a noxious systemic process or a psychogenic driver. Complex cases need real workup: serum electrolytes, upper/lower GI evaluation, fluid input/output tracking, medication history, and screening for behavioral changes, headache, pain, stress, or family history of psychogenic vomiting. Don't keep escalating antiemetic doses on a "simple" framework when the picture is actually complex.
Presentation ranges from mild queasiness to a patient in obvious distress with weight loss and dehydration. Severity, not just presence, is what should drive your workup decision.
| Simple | Complex | |
|---|---|---|
| Course | Self-limiting, resolves spontaneously | Doesn't resolve with standard antiemetics, progressively worsens |
| Signs | Complaint of queasiness or discomfort | Weight loss, fever, abdominal pain |
| Labs needed | None | Serum electrolytes, upper/lower GI eval |
| Cause | Benign, transient | Noxious systemic agent or psychogenic driver |
Persistent vomiting drives hyponatremia, hypokalemia, metabolic alkalosis, and dehydrationfrom lost gastric acid and volume. Any patient with days of vomiting needs those checked before you just keep adding antiemetics on top.
There's no universal diagnostic test for N/V; diagnosis is clinical. What you actually use are risk-prediction scoresthat tell you how aggressive to be with prophylaxis, plus targeted labs when the picture is complex.
Four independent risk factors, each worth one point: female sex, history of PONV or motion sickness, nonsmoking status, and planned postoperative opioid use.The relationship between point count and risk is close to linear and worth memorizing cold.
| Risk factors present | 0 | 1 | 2 | 3 | 4 |
|---|---|---|---|---|---|
| PONV incidence | 10% | 20% | 40% | 60% | 80% |
Translate the count into a risk tier: 0-1 factors = low risk, 2 = moderate risk, ≥3 = high risk.That tier is what determines how many prophylactic drug classes you stack (see PONV section).
A validated scoring tool for nausea/vomiting of pregnancy that quantifies hours of nausea, episodes of vomiting, and episodes of retching over the prior 12-24 hours. Use it to decide when NVP has crossed into something needing pharmacologic escalation versus dietary/behavioral measures alone, and to flag hyperemesis gravidarum for referral.
Goal:prevent or eliminate nausea and vomiting, ideally without adverse effects or with only clinically acceptable ones. Notice the goal is prevention first, not just rescue, especially in CINV and PONV where you know the insult is coming.
One line from lecture worth remembering verbatim in spirit: "treatments are likely most effective if the patient believes they will work."That's not dismissive of CAM options, it's a real reason to still offer sea-bands or ginger to a patient who wants a nondrug option alongside the drug regimen, not instead of counseling that expectation matters.
Once you've identified which of the four input lines is driving the episode, pick the class that blocks it. Simple, infrequent N/V gets small, occasional doses of low-side-effect agents (antacids, first-gen antihistamines). As symptoms persist or the etiology is known to be more severe (CINV, complex PONV), move to combination prescription regimens targeting multiple mechanisms.
Four practical factors guide the actual regimen choice: (1)suspected etiology, (2)frequency/duration/severity of episodes, (3)the patient's ability to tolerate oral vs. rectal vs. injectable vs. transdermal routes, and (4)what's worked (or failed) for that patient before.
| Class / Drug | Adult dose | Notes |
|---|---|---|
| Antihistamines / Anticholinergics (vestibular line) | ||
| Scopolamine (Transderm Scop) | 1.5 mg patch every 72 h | First line for motion sickness |
| Meclizine (Bonine, Antivert) | 12.5-25 mg PO q24h (max 100 mg/day) | 1 hour before travel |
| Dimenhydrinate (Dramamine) | 50-100 mg PO q4-6h (max 400 mg/day) | |
| Diphenhydramine (Benadryl) | 25-50 mg PO/IM/IV q4-6h | |
| Hydroxyzine (Vistaril) | 25-100 mg PO q4-6h | Unlabeled use |
| Dopamine (D2) Antagonists (CTZ line) | ||
| Metoclopramide (Reglan) | 10-20 mg (0.5-2 mg/kg) PO/IV QID | Prokinetic; first line for gastroparesis |
| Prochlorperazine (Compazine/Compro) | 5-10 mg PO/IV q4-6h; 25 mg PR BID | |
| Chlorpromazine (Thorazine) | 10-25 mg PO q4-6h; 25-50 mg IM/IV q4-6h | QT prolongation |
| Promethazine (Phenergan) | 12.5-25 mg PO/IM/IV/PR q4-6h | Contraindicated <2 y/o; tissue injury risk with IV |
| Haloperidol (Haldol) | 0.5-2 mg PO/IM/IV q4-6h | |
| Droperidol (Inapsine) | 0.625-1.25 mg IV; +1.25 mg additional prn | Black box QT/torsades; ECG first |
| Olanzapine (Zyprexa) | 5-10 mg PO daily (5-10 mg days 2-4 for CINV) | Contraindicated with benzodiazepines |
| Amisulpride | 5-10 mg IV (PONV) | Avoid in severe renal impairment |
| 5-HT3 Antagonists (CTZ + gut afferent line) | ||
| Ondansetron (Zofran) | 4-8 mg IV or 8-24 mg PO (dose depends on indication) | Half-life 3-6 h; QT prolongation 1-2 h post-IV |
| Granisetron (Sancuso) | 1-2 mg PO or transdermal patch; 10 mcg/kg IV | Patch lasts up to 24-plus h |
| Palonosetron (Aloxi) | 0.25-0.5 mg IV/PO | Half-life ~40 h, longest acting |
| NK-1 Antagonists (CTZ + vomiting center) | ||
| Aprepitant (Emend) | 125 mg PO day 1, 80 mg days 2-3 (CINV); 40 mg (PONV) | Given at induction for PONV |
| Fosaprepitant | 150 mg IV | IV prodrug of aprepitant |
| Netupitant/palonosetron | 300 mg/0.5 mg PO combo | No additional 5-HT3-RA needed |
| Rolapitant | 180 mg PO (CINV); 70-200 mg (PONV) | CINV only per label |
| Corticosteroid | ||
| Dexamethasone | 4-20 mg PO/IV (dose scales with emetogenic risk) | Single agent or combination |
| Cannabinoids (breakthrough CINV) | ||
| Dronabinol (Marinol) | 5-15 mg/m² PO q2-4h prn | C-III |
| Nabilone (Cesamet) | 1-2 mg PO BID | C-II |
| Benzodiazepine (anticipatory/cortex line) | ||
| Lorazepam (Ativan) | 0.5-2 mg night before and morning of chemo | C-IV; contraindicated with olanzapine |
| NVP-Specific | ||
| Pyridoxine (vitamin B6) | 10-25 mg PO TID-QID | First line in pregnancy |
| Doxylamine | 12.5-20 mg PO up to QID | Add-on to pyridoxine; Diclegis/Bonjesta combo products |
These block H1and M1receptors in the vestibular nuclei, which is exactly the line that fires during motion sickness. Scopolamine is first lineand works because it's a small, highly lipophilic anticholinergic that crosses the blood-brain barrier easily and hits the vestibular apparatus directly.
Wash hands before and after applying (drug on fingers can cause pupil dilation if it touches the eye). Apply to a hairless area behind the ear at least 4 hours beforethe activity, not right before. Press firmly for 30 seconds. Wear only one patch at a time, lasts 72 hours. Rotate to behind the other ear for the next patch. Avoid alcohol (additive drowsiness). Remove before an MRIto prevent a skin burn from the patch's metal backing.
First-generation oral antihistamines (meclizine, dimenhydrinate, diphenhydramine) work the same way but need dosing 30-60 minutes beforethe provocative activity, not after symptoms start. All carry classic anticholinergic baggage: dry mouth, dry eyes, sedation, blurred vision, and in the elderly, confusion, urinary retention, and acute angle-closure glaucoma risk.
Promethazineis technically in this class (antihistaminic) but also has partial antidopaminergic activity, so it carries EPS and neuroleptic malignant syndrome risk that plain meclizine doesn't. It's contraindicated under age 2 (respiratory depression risk) and shouldn't be given IV at concentrations above 25 mg/mL (older dilute formulations above 1 mg/mL raise the same extravasation/tissue injury concern) because of severe tissue necrosis with extravasation.
All of these block D2 receptors at the CTZ. That mechanism is shared with antipsychotics for a reason, these largely arerepurposed antipsychotics, and that's why they all carry the same signature risks: extrapyramidal symptoms (EPS) and neuroleptic malignant syndrome.
Metoclopramidestands apart because it's also a prokinetic, it speeds gastric emptying, which makes it the drug of choice for gastroparesis specifically (where the problem is literally food sitting in the stomach too long from diabetes, scleroderma, lupus, or amyloidosis). It carries a black box warning for tardive dyskinesia with prolonged use, generally limit to under 12 weeks.
Phenothiazines(prochlorperazine, chlorpromazine) are useful for simple N/V or breakthrough CINV, but bring constipation, dizziness, tachycardia, and QTc prolongation.
Droperidol carries a black box warning for QTc prolongation and torsades de pointes. Guidelines call for a 12-lead ECG before administrationand cardiac monitoring for 2-3 hours after. This makes it a poor choice in anyone with baseline QT prolongation, electrolyte derangement, or other QT-prolonging drugs on board.
Olanzapineis the newer addition to this family for CINV; it hits multiple receptors (D2, 5-HT, H1, muscarinic) which is why it's effective for the multi-day nausea window, but it's contraindicated with benzodiazepines(additive sedation/respiratory depression) and needs caution in the elderly.
These block serotonin from binding 5-HT3 receptors both at the CTZ and, critically, on the vagal afferents in the gutwhere enterochromaffin cells release serotonin after mucosal injury from chemo, radiation, or infection. That dual site of action is why they're first-line for both CINV and gastroenteritis-related vomiting.
Adverse effects across the class: headache, constipation, and QT prolongation(ondansetron in particular, occurring 1-2 hours after IV dosing, worth knowing the timing for monitoring). When given in equipotent doses, oral and IV formulations are equally effective, a detail worth remembering when a patient asks why they're being switched to pills.
Palonosetron has a much longer half-life (~40 hours vs. 3-6 hours for ondansetron) and higher receptor-binding affinity, making it the preferred 5-HT3-RA for delayedCINV (the phase starting more than 24 hours after chemo) since a single dose covers the whole risk window.
Pediatric pearl:a single dose of ondansetron in kids with viral gastroenteritis reduces vomiting enough to let oral rehydration therapy succeed, avoiding an IV line and ED admission in many cases.
Substance P acting on NK-1 receptors is a separate signal from both dopamine and serotonin, present at the CTZ andthe vomiting center itself. Blocking it adds a mechanistically independent layer on top of a 5-HT3-RA and steroid, which is why triple therapy beats double therapy in high-emetogenic-risk regimens.
Adverse effects are generally mild: constipation, diarrhea, headache, hiccups, dyspepsia, fatigue. Aprepitant and fosaprepitant are CYP3A4 substrates and moderate inhibitors, so they interact with chemo agents metabolized by that pathway and with dexamethasone itself (which is why the dexamethasone dose gets reduced when co-administered, see the CINV table below).
Rolapitantis labeled only for CINV, not PONV, despite the handbook listing PONV doses; know that the FDA-approved indication is narrower than the trial data might suggest.
Dexamethasoneis useful alone for low-risk prophylaxis or in combination for moderate-to-high risk. Short courses are well tolerated; watch for insomnia, GI upset, agitation, appetite stimulation, hyperglycemia, and hypertension, all of which matter more in diabetic or psychiatric patients getting repeated chemo cycles.
Cannabinoids(dronabinol, nabilone) are reserved for breakthroughCINV that isn't controlled by standard prophylaxis. Adverse effects include euphoria, somnolence, and dry mouth; both are controlled substances (C-III and C-II respectively).
Lorazepamtargets the cortical/limbic input line and is specifically useful for anticipatoryN/V, the classically conditioned nausea that starts before the infusion even begins in patients with a bad prior cycle. It is contraindicated with olanzapine(additive CNS/respiratory depression) and adds sedation on top of any opioid the patient is taking.
CINV has five named categories, and this taxonomy shows up on exams constantly:
| Type | Definition |
|---|---|
| Acute | Onset within 24 hours of chemo administration |
| Delayed | Onset more than 24 hours after chemo |
| Anticipatory (ANV) | Occurs beforethe next chemo dose, a learned/conditioned response |
| Breakthrough | Occurs despite prophylaxis, needs rescue antiemetics; happens in 10-40% of prophylaxed patients |
| Refractory | Poor response to antiemetics in prior cycles despite prophylaxis and rescue |
| Risk tier | Incidence w/o prophylaxis | Example agents | Regimen |
|---|---|---|---|
| High | >90% | Cisplatin, cyclophosphamide >1500 mg/m², anthracycline+cyclophosphamide combo, dacarbazine | NK-1 + 5-HT3 + dexamethasone (± olanzapine); continue prophylaxis through day 3-4 |
| Moderate | 30-90% | Carboplatin, oxaliplatin, ifosfamide, irinotecan, cytarabine >1000 mg/m² | 5-HT3 + dexamethasone; NK-1 optional depending on specific agent |
| Low | 10-30% | Docetaxel, paclitaxel, etoposide, methotrexate, gemcitabine | Choose one: 5-HT3-RA OR dexamethasone alone |
| Minimal | <10% | Bevacizumab, rituximab, vincristine, most immune checkpoint inhibitors | No routine prophylaxis needed |
The duration of emetic risk is 2 days for moderately emetogenic chemo and 3 days for highly emetogenic chemo, and prophylaxis needs to cover the entire window, not just infusion day. Select the regimen based on the single most emetogenic drugin a combination regimen, not an average.
When dexamethasone is combined with an NK-1 antagonist (aprepitant, fosaprepitant, or netupitant/palonosetron), the day-1 dexamethasone dose is cut roughly in half(e.g., 12 mg instead of 20 mg) because those agents inhibit the CYP3A4 enzyme that metabolizes dexamethasone, raising its exposure. This is a real-world dosing detail that trips people up when reading a chemo order set for the first time.
Incidence runs 30-80% depending on risk factors, and it's a real driver of longer hospital stays and cost, not just patient discomfort.
| Category | Factors |
|---|---|
| Patient-related | Age <50, female sex (2-3x higher incidence than males), nonsmoker, prior history of PONV or motion sickness (threefold increase), hydration status |
| Anesthesia-related | General anesthesia, volatile anesthetics, nitrous oxide, intra/postoperative opioids |
| Surgery-related | Laparoscopic, gynecological, or cholecystectomy procedures; longer duration of surgery |
Common combinations: ondansetron + dexamethasone, ondansetron + aprepitant, ondansetron + droperidol, or haloperidol + dexamethasone + ondansetron.
Scopolamine patch:apply the evening before surgery or at least 2 hours prior. NK-1 antagonists:give during induction of anesthesia. Everything else(5-HT3-RAs, dexamethasone, droperidol): give at the end of surgery. Giving a 5-HT3-RA at induction instead of the end of surgery is a common wrong answer choice, since its short half-life means it can wear off before the patient wakes up.
If a patient develops PONV despite prophylaxis with a 5-HT3-RA plus dexamethasone, give rescue therapy from a different drug class(a phenothiazine, metoclopramide, or droperidol), not a repeat dose of what already failed. If no prophylaxis was given at all, first-line rescue is ondansetron 4 mg PO or IV.
Extremely common: 50-80% of pregnancies have nausea, ~50% have vomiting.Onset is typically before 9 weeks gestation, usually resolving by week 14, though some patients have symptoms throughout pregnancy. Risk factors include multiple gestations, prior motion sickness, migraine history, family history, and a personal history of hyperemesis gravidarum.
Patients with persistent NVP or dehydration who need IV fluids should get thiamine before dextrose, not the other way around. Dextrose given to a thiamine-depleted patient can precipitate Wernicke encephalopathy. This is a real inpatient order-set error, not just trivia; always check that thiamine is on the fluid order before dextrose runs.
Ondansetron, promethazine, and metoclopramide have similar effectiveness for hyperemesis gravidarum, but ondansetron tends to be better tolerated with fewer adverse effects, which is why it's frequently reached for even though it isn't the labeled first-line agent. Weigh that against the historical (and largely unresolved) discussion around first-trimester cardiac malformation signal when counseling.
Wernicke encephalopathy, splenic avulsion, esophageal rupture, pneumothorax, and acute tubular necrosis. These are the consequences of prolonged forceful vomiting plus nutritional depletion, and they're why hyperemesis gravidarum is treated as a real admission-worthy diagnosis rather than "bad morning sickness."
Any patient on carbidopa/levodopa or with Parkinson's disease should avoid metoclopramide, prochlorperazine, promethazine, chlorpromazine, and other central dopamine antagonists.These drugs cross into the CNS and directly antagonize the same dopamine receptors that levodopa is trying to stimulate, worsening motor symptoms and potentially causing acute EPS on top of existing parkinsonism. Ondansetron or trimethobenzamideare the safer choices since they don't have central D2 activity. This exact scenario (a postoperative PD patient on carbidopa/levodopa needing an antiemetic) is a classic case-based question.
Drugs that prolong QTc and deserve a baseline ECG check or caution when stacked together: ondansetron (especially IV), droperidol (black box), haloperidol, and chlorpromazine.Combining two of these, or giving them with other QT-prolonging home medications, is a preventable medication error worth catching at order verification.
Any central dopamine antagonist can cause extrapyramidal symptoms or, rarely, neuroleptic malignant syndrome: metoclopramide, prochlorperazine, chlorpromazine, promethazine (partial), haloperidol, droperidol, olanzapine.Watch for it especially with repeated dosing or in patients already on antipsychotics.
Antihistaminic-anticholinergic agents are especially problematic in older adults: increased risk of confusion, falls, urinary retention, and acute angle-closure glaucoma. Use extra caution in patients with benign prostatic hyperplasia, narrow-angle glaucoma, or asthma, since anticholinergic effects worsen all three.
| Parameter | When | Watching for |
|---|---|---|
| Symptom resolution | Every dose/regimen, ongoing | Whether the chosen mechanism actually matched the trigger |
| Hydration status | Any patient with more than a day or two of vomiting | Dehydration, orthostasis, reduced urine output |
| Electrolytes (Na, K, bicarb) | Complex or persistent N/V | Hyponatremia, hypokalemia, metabolic alkalosis |
| QTc / ECG | Before droperidol; consider with high-dose ondansetron, haloperidol, chlorpromazine | QT prolongation, torsades risk |
| Extrapyramidal symptoms | Ongoing with any D2 antagonist, especially repeated dosing | Dystonia, akathisia, parkinsonism, tardive dyskinesia |
| Sedation level | Anticholinergics, benzodiazepines, olanzapine, especially combined | Additive CNS/respiratory depression, fall risk in elderly |
| Weight / ketonuria | NVP and hyperemesis gravidarum | Progression toward hyperemesis, nutritional status |
| Blood glucose | Repeated or prolonged dexamethasone courses | Steroid-induced hyperglycemia, especially in diabetics |