What are the side effects of ondansetron in an adult patient with persistent post‑viral nausea, fatigue, and dizziness secondary to omeprazole?

Side Effects of Ondansetron

Ondansetron is generally well tolerated, with the most common side effects being headache (occurring in 9-24% of patients), constipation (9%), diarrhea (4-6%), dizziness (7%), and fatigue (13%). 1, 2

Common Side Effects (Occurring in >2% of Patients)

Gastrointestinal Effects

• Constipation occurs in approximately 9% of patients and can paradoxically worsen nausea if not addressed proactively 2, 1
• Diarrhea affects 4-6% of patients, creating a bidirectional gastrointestinal effect profile 1, 3
• Transient elevations in liver enzymes (AST/ALT exceeding twice the upper limit of normal) occur in 1-2% of patients, though these are typically transient and asymptomatic 1

Neurological Effects

• Headache is the most frequent side effect, reported in 9-24% of patients depending on the indication and dose 1, 3
• Dizziness occurs in approximately 7% of patients 1
• Fatigue/malaise affects 13% of patients 1
• Sedation can occur, though less commonly than with first-generation antihistamines 4

Cardiovascular Effects (Critical Safety Concern)

• QT interval prolongation is a serious risk that requires baseline ECG monitoring before initiating therapy, particularly in patients taking other QT-prolonging medications 2, 1
• Myocardial ischemia has been reported, predominantly with intravenous administration but can occur with oral dosing; coronary artery spasm appears to be the underlying mechanism 1
• Rare arrhythmias including ventricular tachycardia, supraventricular tachycardia, atrial fibrillation, and bradycardia have been documented 1, 5
• Palpitations and syncope occur rarely 1

Rare but Serious Side Effects (<1% Incidence)

Hypersensitivity Reactions

• Anaphylaxis, angioedema, bronchospasm, and laryngeal edema can occur 1
• Rash and pruritus affect approximately 1-5% of patients 1
• Laryngospasm, shock, and cardiopulmonary arrest have been reported during severe allergic reactions 1

Neurological

• Extrapyramidal reactions occur in less than 1% of patients—significantly lower than with dopamine antagonists like metoclopramide 1, 4
• Grand mal seizures have been reported rarely 1

Other Rare Effects

• Fever (pyrexia) in 8% of postoperative patients 1
• Urinary retention in 5% of postoperative patients 1
• Hiccups 1
• Flushing 1

Critical Safety Warnings

Masking of Serious Conditions

• Ondansetron can mask progressive ileus and gastric distension, particularly in post-abdominal surgery patients or those receiving chemotherapy 1
• The drug does not stimulate gastric or intestinal peristalsis and should never replace nasogastric suction when indicated 1

Serotonin Syndrome Risk

• When combined with other serotonergic drugs (including SSRIs like the omeprazole context suggests), there is an increased risk of serotonin syndrome 1
• Monitor for agitation, confusion, tachycardia, hypertension, hyperthermia, hyperreflexia, and incoordination 1

Drug-Specific Context for Your Patient

• In a patient already experiencing dizziness and fatigue, ondansetron may worsen these symptoms (7% dizziness rate, 13% fatigue rate) 1
• The constipation side effect (9%) is particularly relevant since this can create a feedback loop worsening nausea 2, 1
• If the patient is taking omeprazole for reflux, the combination addresses potential dyspepsia that can mimic nausea, but ondansetron's constipating effect may counteract omeprazole's benefits 2

Tolerability Profile Compared to Alternatives

• Ondansetron has a significantly lower incidence of extrapyramidal side effects compared to dopamine antagonists like metoclopramide (<1% vs. higher rates with metoclopramide) 1, 4
• The drug is generally well tolerated across all age groups, though elderly patients may have slightly prolonged half-life (3.5 hours in adults) 4
• In prehospital settings, ondansetron demonstrated excellent safety with minimal adverse effects in over 2,000 patients 6