How long after starting Lexapro (escitalopram) would I know if I will develop QT prolongation?

Timeline for Detecting QT Prolongation on Lexapro (Escitalopram)

You should obtain a repeat ECG at 2 weeks after starting Lexapro to determine if QT prolongation will develop, as this is when drug-induced QT changes typically manifest. 1

Initial Baseline Assessment

Before starting Lexapro, you must establish your baseline cardiac status:

• Obtain a baseline ECG to measure your QTc interval using Fridericia's formula, with normal values being <430 ms in males and <450 ms in females 1
• Do not start Lexapro if your baseline QTc is >500 ms, as this represents an absolute contraindication 1
• Check electrolytes (potassium and magnesium) and correct any abnormalities before initiating treatment, maintaining potassium >4.0 mEq/L 1

Critical Monitoring Window

The key timeframe for detecting QT prolongation is within the first 2 weeks of treatment:

• Repeat ECG at 2 weeks after starting Lexapro—this is the standard monitoring interval recommended to detect drug-induced QT changes 1
• The FDA label data shows that at therapeutic doses (10-20 mg), escitalopram causes a mean QTc prolongation of 4.5-10.7 ms, which would be evident by this timepoint 2
• Research on overdose cases demonstrates that QT prolongation peaks within 18-48 hours of exposure and begins normalizing after 2-3 days, suggesting that therapeutic dosing effects stabilize within the first 1-2 weeks 3, 4

Ongoing Monitoring Requirements

After the initial 2-week check, continue surveillance at specific intervals:

• Repeat ECG after any dose increase, as higher doses (20 mg vs 10 mg) produce greater QT prolongation 1, 2
• Repeat ECG when adding any new QT-prolonging medication, as drug interactions significantly amplify risk 1, 5
• Monitor electrolytes regularly, particularly if you develop diarrhea (a common Lexapro side effect that can cause electrolyte depletion) 6, 1

When to Stop Lexapro

Discontinue escitalopram immediately if:

• QTc exceeds 500 ms at any point during treatment 6, 1
• QTc increases >60 ms from your baseline value 6, 1
• You develop symptoms of arrhythmia (palpitations, syncope, dizziness) 6

High-Risk Situations Requiring Extra Vigilance

You face increased risk if you have:

• Cardiac risk factors including heart disease, bradycardia, or family history of sudden death 1
• Concomitant medications that prolong QT, particularly antipsychotics, certain antibiotics (macrolides, fluoroquinolones), or Class IA/III antiarrhythmics 1, 5, 7
• Genetic susceptibility through polymorphisms in KCNE1 (rs1805127 C allele, rs4817668 C allele) or KCNH2 (rs3807372) genes, though routine genetic testing is not standard practice 7
• Advanced age or multiple comorbidities, as elderly patients show more pronounced QT prolongation even at recommended doses 8

Practical Considerations

The FDA label indicates that none of the patients on therapeutic doses of escitalopram developed QTc >500 ms or prolongation >60 ms in controlled trials, compared to 0.2% of placebo patients 2. This suggests that clinically significant QT prolongation at standard doses (10-20 mg) is uncommon in patients without additional risk factors. However, the 2-week ECG remains essential because individual susceptibility varies based on genetics, drug interactions, and underlying cardiac conditions 7.

If your 2-week ECG shows QTc remains <500 ms and the change from baseline is <60 ms, you can be reasonably confident that Lexapro is not causing dangerous QT prolongation at your current dose 6, 1.