There is no published answer to this question in days. No trial has ever reported a day-by-day onset for semaglutide, so every “week 1 you’ll feel this, week 2 you’ll feel that” timeline online was written rather than measured.
What the FDA labels and the published trials do give you are five separate timings, and most of the confusion about this drug comes from treating them as one number.
The drug is in your blood within days — and still climbing for a month
The Wegovy label puts maximum concentration at 1 to 3 days after a subcutaneous dose, with an absolute bioavailability of 89%. So semaglutide is present and active within the first few days of the very first injection.
That is not the same as being at full strength. The Ozempic label states that steady-state exposure is achieved following 4 to 5 weeks of once-weekly administration. Until then each injection lands on top of the remainder of the last one and the average level in your blood is still rising. On the same 0.25 mg dose, week 4 is a meaningfully higher exposure than week 1.
This is also why the titration steps are 4 weeks apart rather than weekly — a dose has not finished showing what it does until roughly the point the next step is due.
When appetite was actually measured
Two trials measured appetite and food intake directly rather than inferring it from weight. Both are worth knowing precisely, because the weeks they chose are the earliest published evidence that exists.
- Week 12, semaglutide 1.0 mg. A 30-subject crossover trial found ad libitum energy intake was lower at lunch (−1255 kJ), at the following evening meal and for snacks, giving a 24% reduction in total energy intake across all ad libitum meals in the day (−3036 kJ). Participants reported less hunger, fewer food cravings, better control of eating and a lower preference for high-fat food. Resting metabolic rate, adjusted for lean mass, did not differ from placebo.
- Week 20, semaglutide 2.4 mg. A 72-adult trial found ad libitum energy intake 35% lower than placebo — 1736 kJ versus 2676 kJ — with reduced hunger and prospective food consumption and increased fullness and satiety. Body weight fell 9.9% versus 0.4% on placebo. Notably, there was no evidence of delayed gastric emptying at week 20 when assessed by paracetamol absorption, despite the label listing delayed gastric emptying as a pharmacodynamic effect.
Neither trial published what was happening at week 1, week 2 or week 4. That gap is the honest answer to “how long until it suppresses appetite”: the effect is large and well-documented by week 12, and its arrival was never timed.
Why the first four months are not a test of the drug
The escalation schedule in the Wegovy label exists to reduce the risk of gastrointestinal adverse reactions, not to find an effective dose. It runs 0.25 mg for weeks 1–4, 0.5 mg for weeks 5–8, 1 mg for weeks 9–12, 1.7 mg for weeks 13–16, and only from week 17 the maintenance dose of 1.7 mg or 2.4 mg. The 7.2 mg dose is available only after at least 4 more weeks of tolerating 2.4 mg.
So a patient asking at week 6 whether it is working has had one week at a dose above the starting step, and is roughly at the point where blood levels first stabilise. If a step is not tolerated, the label’s instruction is to consider delaying escalation by 4 weeks rather than pushing through, which extends the runway further.
What the weight numbers actually say
The 68-week trial published as STEP 1 and reproduced in the Wegovy label as Study 2 reported a mean body weight change of −14.9% on semaglutide 2.4 mg against −2.4% on placebo, from a baseline of about 105 kg. 83.5% of patients lost at least 5% of body weight, 66.1% lost at least 10%, 47.9% lost at least 15% and 30.2% lost at least 20%.
In patients who also had type 2 diabetes, the same dose gave −9.6% against −3.4% on placebo — consistently less weight loss than in patients without diabetes, which is worth knowing before comparing yourself to a headline figure.
The label reports these at week 68 and shows the time course only as a figure, without publishing the intermediate means. It does not state a week at which weight loss begins or plateaus.
The only GLP-1 label that sets a deadline
Neither semaglutide label tells a prescriber when to give up. The liraglutide label does, and it is the closest thing to an official answer to “how long before I know?”: evaluate the change in body weight 16 weeks after starting Saxenda and discontinue it if the patient has not lost at least 4% of baseline body weight, since it is unlikely that they will achieve and sustain clinically meaningful weight loss with continued treatment.
That is a rule for a different drug on a different schedule, so it does not transfer mechanically. It is quoted here because it is the only 16-week checkpoint any GLP-1 label actually publishes, and because a 4% threshold is far lower than most people expect.
Related
- How long a GLP-1 stays in your system — the other direction: half-lives, clearance, and why nothing speeds it up.
- Semaglutide dosage calculator and full titration schedule
- Is semaglutide a peptide? — the structural reason its half-life is a week rather than minutes.