Drug Level Calculator
Enter your medication and the days since your last injection to see your current drug level and clearance timeline.
GLP-1 Half-Lives: All Medications
Half-life is the time it takes for the concentration of a drug in the body to reduce by 50%. Most drugs require 5 half-lives to be considered fully eliminated (less than 3% remaining).
| Medication | Active Ingredient | Half-Life | Full Clearance | Dosing Frequency |
|---|---|---|---|---|
| Ozempic / Wegovy / Rybelsus | Semaglutide | ~7 days | ~35 days | Once weekly |
| Mounjaro / Zepbound | Tirzepatide | ~5 days | ~25 days | Once weekly |
| Victoza / Saxenda | Liraglutide | ~13 hours | ~3 days | Once daily |
| Trulicity | Dulaglutide | ~5 days | ~25 days | Once weekly |
| Bydureon BCise | Exenatide ER | ~2 weeks (microspheres) | ~10 weeks | Once weekly |
💡 Semaglutide's exceptionally long 7-day half-life is by design — it was engineered with albumin-binding modifications that protect it from degradation and extend its activity. This is what makes once-weekly dosing possible and also why it remains detectable for 5+ weeks after the last dose.
Why Half-Life Matters in Practice
Missed Doses
Because semaglutide and tirzepatide have long half-lives (5–7 days), a single missed dose doesn't cause your drug level to drop to zero. After one missed injection, approximately 50% of the previous dose's level remains in your system. This is why most guidelines allow up to 5 days late for a missed semaglutide dose before needing to adjust timing.
Surgery and Procedures
GLP-1 medications slow gastric emptying — a critical concern for surgery. Even if you've fasted appropriately, residual food in the stomach due to delayed gastric emptying increases aspiration risk during anesthesia. Most anesthesiology societies now recommend:
- Semaglutide (Ozempic/Wegovy): Stop 1–2 weeks before elective surgery minimum; some guidelines recommend 4 weeks before major procedures
- Tirzepatide (Mounjaro/Zepbound): Stop 1–2 weeks before elective procedures
- Liraglutide (Saxenda): Stop 1–2 days before surgery (shorter half-life)
- Always follow your surgical team's specific instructions
🚨 Do not stop your GLP-1 medication before surgery without informing and getting approval from both your prescribing provider AND your surgical/anesthesia team. Stopping abruptly can affect blood sugar control in diabetic patients.
Stopping the Medication
When you stop a GLP-1 medication, drug levels decline according to the half-life curve. Most people notice:
- Nausea and GI side effects: Improve within 1–2 weeks of the last dose
- Appetite suppression: Noticeably decreased by week 2–3; hunger usually returns to pre-treatment levels by week 4–8
- Blood sugar effects (in T2D patients): Begin reverting toward baseline within 2–4 weeks
- Weight regain: Typically begins within 4–8 weeks and accelerates without intervention
Drug-Drug Interactions and Timing
Because GLP-1 medications slow gastric emptying, they can delay the absorption of oral medications. Particularly relevant for:
- Oral contraceptives (take at least 1 hour before or 4 hours after GLP-1 injection day)
- Thyroid medications (levothyroxine) — take separately, preferably in the morning fasted
- Metformin and other oral diabetes drugs — monitor for enhanced blood sugar effects
- Warfarin — monitor INR more frequently when starting/stopping GLP-1 therapy
Semaglutide Half-Life: Deep Dive
Native GLP-1 has a half-life of just 2 minutes — it's degraded almost instantly by the enzyme DPP-4. Semaglutide overcomes this through albumin binding, structural modifications with a fatty acid chain, and amino acid substitutions that prevent enzymatic breakdown. The result is a molecule that maintains therapeutic drug levels throughout the full 7-day dosing interval with minimal peak-to-trough variation.
| Days After Last Semaglutide Dose | Approx % Remaining | Clinical Significance |
|---|---|---|
| Day 0 (injection day) | 100% | Full therapeutic level |
| Day 7 (next injection due) | ~50% | Still therapeutically active |
| Day 14 | ~25% | Reduced but detectable effect |
| Day 21 | ~12% | Minimal clinical effect |
| Day 35 | ~3% | Effectively cleared |
| Day 42 | <1% | Fully eliminated |