Nursing care
Antiemetics: what to check before you give it
Written and reviewed by Dana Whitfield, RN, MSN · 4 min read · Updated September 2026
Short answer
Antiemetics prevent or relieve nausea and vomiting by blocking receptors involved in the vomiting reflex, most commonly serotonin (5-HT3), dopamine, or histamine pathways. Before giving one, check the baseline ECG and electrolytes if the drug is ondansetron, since it prolongs the QT interval — a risk not obvious from its use as a routine anti-sickness drug.
What it does and why it is prescribed
Antiemetics work by blocking one of the pathways that trigger the vomiting reflex. Ondansetron and other setrons block serotonin at 5-HT3 receptors in the gut and the chemoreceptor trigger zone. Promethazine blocks histamine and has anticholinergic and dopamine-blocking effects, which is why it also sedates. Metoclopramide blocks dopamine and speeds gastric emptying, which makes it useful in gastroparesis as well as nausea.
They are prescribed for postoperative nausea and vomiting, chemotherapy-induced nausea, hyperemesis gravidarum, motion sickness, and nausea from opioids or other medications. The choice of agent depends on the cause. A patient vomiting from chemotherapy needs a different mechanism blocked than one with vertigo-related nausea, so matching the drug to the trigger matters more than reaching for whatever is on the ward stock list.
Nursing considerations before giving it
Before administering ondansetron, check the baseline ECG if the patient has a cardiac history, is on other QT-prolonging drugs, or has electrolyte disturbance. Correct low potassium or low magnesium first where possible, since both increase the risk of torsades de pointes on top of a prolonged QT. This is the check that catches nurses out, because ondansetron is used so routinely that its cardiac risk gets forgotten.
For promethazine, confirm the route and site carefully. It is caustic to tissue and IV administration carries a real risk of severe tissue injury if it extravasates, so central line administration is preferred where policy allows, and the site should be checked frequently during infusion. For metoclopramide, ask about a history of Parkinson's disease or extrapyramidal reactions before giving it, since dopamine blockade can worsen both.
What to monitor
With ondansetron, monitor the ECG or at minimum stay alert to palpitations, dizziness, or syncope, particularly with IV doses above 16 mg or rapid administration. Watch electrolytes throughout, especially in patients who are also vomiting or on diuretics, since ongoing losses compound the QT risk rather than resolving it after one correction.
With promethazine, monitor sedation level and respiratory rate, especially in older adults or anyone on other CNS depressants. With metoclopramide, monitor for restlessness, tremor, or abnormal movements, which can signal an extrapyramidal reaction and usually need the drug stopped rather than managed through.
Side effects versus adverse effects
Expected side effects are usually mild and dose-related: headache and constipation with ondansetron, drowsiness and dry mouth with promethazine, restlessness with metoclopramide. These are uncomfortable but not dangerous, and patients can usually be reassured and monitored.
Adverse effects are the ones that change the plan of care. QT prolongation and torsades de pointes with ondansetron are adverse effects, not side effects, because they are potentially fatal and require stopping the drug. Extrapyramidal symptoms and neuroleptic malignant syndrome with metoclopramide, and severe respiratory depression with promethazine, fall into the same category. The distinction is not severity alone — it is whether the reaction is an expected nuisance or a signal to act.
What to hold for and when to call
Hold ondansetron and call the prescriber if the baseline QTc is already prolonged, if potassium or magnesium is low and uncorrected, or if the patient reports new palpitations or dizziness after a dose. Hold promethazine in children under two, where it carries a risk of fatal respiratory depression, and use caution with any patient with compromised respiratory status.
Hold metoclopramide and call if the patient develops involuntary movements, and stop it outright if there is any suggestion of neuroleptic malignant syndrome — rigidity, hyperthermia, autonomic instability. In all cases, escalate rather than substitute another antiemetic without checking, since several classes share overlapping risks.
Patient teaching
Tell patients taking ondansetron at home to report palpitations, fainting, or an irregular heartbeat, and to mention any heart condition or other medications before starting it, since drug interactions compound the QT risk. Advise against driving after promethazine until they know how sedated it makes them, and warn that it can intensify the effect of alcohol.
For metoclopramide, tell patients to report any new muscle stiffness, tremor, or restlessness promptly rather than waiting it out, and to avoid taking it for longer than the prescribed short course, since extrapyramidal risk rises with cumulative dose and duration.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our gastrointestinal practice questions are the closest set to what this page covers.
Common questions
Why does ondansetron need ECG monitoring?
Ondansetron blocks cardiac potassium channels and prolongs the QT interval, which can lead to torsades de pointes in susceptible patients. The risk rises with IV dose, rapid administration, low potassium or magnesium, and other QT-prolonging drugs, so baseline ECG and electrolyte checks are part of safe administration, not an optional extra.
Can promethazine and ondansetron be given together?
They can be, and are sometimes combined when a single agent isn't controlling nausea, but each carries its own risk profile that still needs checking independently. Sedation from promethazine and QT risk from ondansetron do not cancel each other out — both need monitoring.
What is the antidote for extrapyramidal symptoms from metoclopramide?
Diphenhydramine or benztropine is typically given to reverse acute extrapyramidal reactions such as dystonia. The metoclopramide should also be stopped and the reaction documented, since the risk recurs with re-exposure.
Is ondansetron safe in pregnancy?
It is widely used for hyperemesis gravidarum, but evidence on first-trimester risk has been mixed across studies, so prescribers weigh the severity of maternal symptoms against the uncertainty. This is a shared decision between the patient and prescriber rather than a fixed rule, and nursing practice should follow local protocol.