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What to Expect

How long until a GLP-1 starts working?

Updated 2026-05-05

Appetite changes show up fast - most people notice they eat less by week 1 or 2. Real weight loss takes longer. In the STEP 1 trial of semaglutide, the average loss was about 5.9 kg by week 12 and 13.7 kg by week 68. In the SURMOUNT-1 trial of tirzepatide, the highest dose group lost about 22.5% body weight by week 72. The big drop comes from staying on the drug for a year, not from the first month.

IfIf the scale has not moved after 4 weeks
Thenthen check your protein intake and steps - the drug is working on appetite, you have to do the rest
IfIf you feel zero appetite change after 6 weeks
Thenthen talk to your prescriber about a dose increase or switching to tirzepatide
IfIf you lose 5+ pounds in week 1
Thenthen most of that is water - the real fat loss curve is slower and steadier
IfIf you stop early because progress feels slow
Thenthen know that quitting before week 12 means most of the trial-level weight loss never happens
Key facts
  • STEP 1 (semaglutide 2.4 mg weekly): mean weight loss 14.9% at week 68, with about 6 kg lost by week 12
  • SURMOUNT-1 (tirzepatide 15 mg weekly): mean weight loss 22.5% at week 72
  • Both drugs use a slow titration (4-week dose steps) to reduce nausea, which delays peak effect
  • Appetite suppression typically begins within 1-2 weeks at the starting dose
  • Plateaus around weeks 8-16 are common as the body adjusts before the next dose step
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