I accidentally took a double dose of hydroxychloroquine (400 mg) today; as a healthy adult without cardiac disease, electrolyte abnormalities, or QT‑prolonging medications, what symptoms should I monitor for and when should I seek medical evaluation?

Accidental Double Dose of Hydroxychloroquine (400 mg)

A single accidental double dose of hydroxychloroquine (400 mg total) in a healthy adult without cardiac disease, electrolyte abnormalities, or QT-prolonging medications is unlikely to cause serious toxicity, but you should monitor for gastrointestinal symptoms and cardiac manifestations over the next 24-48 hours.

Immediate Risk Assessment

Your risk profile is favorable because:

• Acute severe toxicity typically requires much higher doses (therapeutic overdoses causing mortality involve substantially higher amounts than 400 mg) 1, 2
• You lack high-risk features including pre-existing cardiac disease, electrolyte abnormalities, and concurrent QT-prolonging medications 3, 4
• Toxic symptoms from hydroxychloroquine overdose manifest within 1-3 hours of ingestion 5

Symptoms to Monitor (Next 24-48 Hours)

Gastrointestinal Effects (Most Likely)

• Nausea, vomiting, diarrhea, and stomach discomfort are the most common adverse effects and may occur within hours 6, 2
• These symptoms typically resolve spontaneously and do not indicate serious toxicity 6

Cardiac Manifestations (Critical to Recognize)

Monitor for:

• Palpitations, chest discomfort, or sensation of irregular heartbeat 3, 7
• Lightheadedness, dizziness, or near-syncope (not vasovagal, but potentially from conduction abnormalities or arrhythmias) 3, 1
• Severe hypotension symptoms including profound weakness or confusion 5, 1

Neurological Symptoms

• Headache (common and typically benign) 6
• Confusion, disorientation, visual disturbances, or seizures (rare but serious, warrant immediate evaluation) 5, 2

When to Seek Immediate Medical Evaluation

Go to the emergency department immediately if you develop:

• Any cardiac symptoms including palpitations, chest pain, syncope, or severe lightheadedness 3, 1
• Neurological symptoms including confusion, visual changes, seizures, or altered consciousness 5, 2
• Severe or persistent vomiting that prevents oral intake 2
• Any symptoms that develop within the first 3 hours after ingestion, as this is the typical window for acute toxicity manifestation 5, 1

Routine Monitoring Recommendations

If you remain asymptomatic or develop only mild gastrointestinal symptoms:

• Continue observation for 24-48 hours as neuropsychiatric symptoms can manifest within this timeframe 2
• Resume your regular hydroxychloroquine dose tomorrow (do not skip or double-adjust) 8
• Contact your prescribing physician to report the incident for documentation 6

Key Clinical Context

Why this dose is unlikely to cause serious harm:

• Standard therapeutic dosing is 200 mg twice daily (400 mg/day total), so your accidental dose equals one day's therapeutic amount taken at once 8
• Acute severe toxicity and mortality occur with substantially higher overdoses, not with therapeutic dose ranges 1, 2
• Cardiovascular collapse, severe arrhythmias, and seizures are associated with intentional overdoses far exceeding therapeutic amounts 5, 1

Important Caveats

Do not assume all symptoms are benign:

• Cardiac conduction abnormalities can be the initial manifestation of hydroxychloroquine toxicity and should never be dismissed 3, 7
• Hypokalemia can develop acutely and compounds cardiac risk, though this is more common with higher overdoses 4, 5
• If you develop any cardiac symptoms, obtain a 12-lead ECG immediately to assess for QRS widening, QTc prolongation >500 ms, or conduction blocks 3, 4