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What to Expect

How long until a GLP-1 starts working?

Updated 2026-05-05

Appetite changes tend to show up fast, and many people report eating less within the first week or two. 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 over the long haul, not from the first month. Talk to your provider about the timeline and plan that make sense for you.

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 bring it up with your prescriber - they can look at whether your plan still fits
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): mean weight loss about 14.9% at week 68, with roughly 6 kg lost by week 12
  • SURMOUNT-1 (tirzepatide): mean weight loss about 22.5% at week 72
  • Both drugs are titrated up gradually to reduce nausea, which delays the peak effect
  • Appetite suppression is often reported within the first week or two
  • Plateaus around weeks 8-16 are common as the body adjusts
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