When do GLP-1 side effects start?
Semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro, Zepbound) are injected once a week and started at a deliberately low dose that is raised in steps: Wegovy goes 0.25 → 0.5 → 1 → 1.7 → 2.4 mg with each step lasting four weeks, so the maintenance dose arrives at week 17; Zepbound starts at 2.5 mg for four weeks and rises in 2.5 mg increments no faster than every four weeks, with a 20-week escalation period in its pivotal trial. Both labels state the same reason for the slow ramp: to reduce gastrointestinal side effects.
That schedule is the key to the timeline. Side effects on a GLP-1 do not arrive as one long block; they arrive as a wave after each dose step, strongest in the first days after the injection, easing as the body adapts — and then a new step brings a new, usually smaller wave. The pooled STEP 1–3 analysis by Wharton and colleagues found gastrointestinal events "occurred most frequently during/shortly after dose escalation", and the Zepbound label says the same: the majority of nausea, vomiting and diarrhoea events occurred during dose escalation and decreased over time.
Which GLP-1 side effects are most common?
The two U.S. prescribing labels report side effects from the placebo-controlled weight-loss trials. These are the reactions reported by at least 2% of people and more often than on placebo:
| Side effect | Wegovy 2.4 mg (n=2,116) | Placebo | Zepbound 15 mg (n=941) | Placebo |
|---|---|---|---|---|
| Nausea | 44% | 16% | 28% | 8% |
| Diarrhoea | 30% | 16% | 23% | 8% |
| Vomiting | 24% | 6% | 13% | 2% |
| Constipation | 24% | 11% | 11% | 5% |
| Abdominal pain | 20% | 10% | 10% | 5% |
| Fatigue | 11% | 5% | 7% | 3% |
| Dyspepsia (indigestion) | 9% | 3% | 10% | 4% |
| Dizziness | 8% | 4% | 4% | 2% |
| Burping (eructation) | 7% | <1% | 5% | 1% |
| Hair loss | 3% | 1% | 5% | 1% |
Two cautions before you read across the columns. The labels themselves warn that rates from different trials cannot be compared directly — the populations, definitions and durations differ — so this table does not show that one drug is gentler than the other. And "reported nausea" counts anyone who mentioned it once in 68–72 weeks; it says nothing about how bad or how long. For that, the Wharton analysis is more useful: of the gastrointestinal events on semaglutide, 99.5% were non-serious and 98.1% mild-to-moderate.
GLP-1 side effects timeline: week by week
Weeks 1–4: the starting dose
Appetite drops almost immediately, and with it the first side effects: feeling full after a few bites, nausea, burping, and in some people either loose stools or the opposite. The starting dose (0.25 mg Wegovy, 2.5 mg Zepbound) is not a treatment dose — both labels say it is for initiation only — so this month is about tolerability, not results. Symptoms typically cluster in the two or three days after each injection.
Weeks 5–16 (Wegovy) or 5–20 (Zepbound): dose escalation
This is the window where most gastrointestinal events happen and where most people who quit, quit. The Zepbound label notes that the majority of patients who discontinued because of side effects "did so during the first few months of treatment due to gastrointestinal adverse reactions"; in the pooled Wegovy trials 6.8% of participants stopped for adverse reactions versus 3.2% on placebo, led by nausea (1.8%), vomiting (1.2%) and diarrhoea (0.7%). Each 4-week step tends to bring a fresh wave that settles within the step. The Wegovy label also builds in a safety valve — if a dose is not tolerated during escalation, the prescriber can delay the next step by four weeks — which is exactly why a dated log of symptom severity is worth more than a vague "it's been rough".
Weeks 17–24: the first weeks at maintenance dose
For most people the gastrointestinal wave recedes here. The best evidence is STEP 4, which took 803 people who had tolerated a 20-week semaglutide run-in and kept them on 2.4 mg: maintenance was "well tolerated". Constipation deserves a separate note. Both labels tie the "peaks during escalation, then fades" pattern specifically to nausea, vomiting and diarrhoea — constipation is not on that list, so do not assume it will resolve on its own. It is common (24% on Wegovy), and fluids and fibre are the first conversation to have with your prescriber or a dietitian.
Months 3–6: the weight-loss side effects
A second group of effects is not caused by the drug's action on the gut but by the speed of weight loss itself, and it shows up later:
- Hair loss. Both labels describe hair loss as "associated with weight reduction" — Wegovy 3.3% versus 1% on placebo; on Zepbound 7.1% of women versus 0.5% of men reported it, against 1.3% and 0% on placebo. Dermatology reviews describe this pattern (telogen effluvium) as diffuse shedding that begins 3–4 months after a trigger, which is why it surprises people who felt fine during escalation. No Zepbound participant stopped treatment because of it.
- Gallstones. Rapid weight loss raises gallstone risk on any diet; the Wegovy label reports cholelithiasis in 1.6% versus 0.7% on placebo, and notes the excess persisted "even after accounting for the degree of weight loss". Zepbound: cholelithiasis 1.1% versus 1%, cholecystitis 0.7% versus 0.2%.
- Facial volume loss — "Ozempic face" — is the same mechanism; we cover it in a separate guide.
Month 6 and beyond
At a stable dose the picture is mostly quiet, with two things worth knowing. Resting heart rate rises by a mean of 1–4 beats per minute on Wegovy — routine, but visible if you track it. And the dehydration risk never fully goes away: the acute-kidney-injury warning on both labels is driven by people who kept losing fluid to vomiting or diarrhoea without replacing it. Fatigue and dizziness at this stage are often an intake problem — too little food and water for the day you are actually having — rather than a drug problem, and a log makes the difference obvious.
Which side effects are not "normal" — when should you call your prescriber?
The labels list a short set of reactions that need a call the same day, not a wait-and-see:
- Severe, persistent abdominal pain, sometimes radiating to the back, with or without vomiting — the warning sign for pancreatitis, which both labels say requires stopping the drug if suspected.
- Pain in the upper right abdomen, fever, or yellowing skin/eyes — possible gallbladder disease.
- Vomiting or diarrhoea you cannot keep ahead of with fluids — the route to dehydration and acute kidney injury.
- Low blood sugar if you also take insulin or a sulfonylurea: in the Zepbound type 2 diabetes trial, hypoglycaemia occurred in 10.3% of people on a sulfonylurea versus 2.1% without one. Your prescriber may need to adjust the other medication — not you.
- Swelling of the face, lips or throat, or a rash with breathing difficulty — hypersensitivity, reported post-marketing with both drugs.
- Any change in vision if you have type 2 diabetes and a history of retinopathy.
On mood: in February 2026 the FDA removed the "suicidal behaviour and ideation" warning from both labels after review. That is reassuring at the population level, but it does not mean your mood is irrelevant — track it, and read what the data actually shows on GLP-1 and mental health.
Do side effects mean the drug is working better?
No — and this is one of the cleaner findings in the whole literature. In the pooled STEP 1–3 data, people on semaglutide who had no gastrointestinal side effects lost 9.6–17.1% of body weight; people who did have them lost 11.4–17.7%. A formal mediation analysis found that of the extra 7.6–14.4% weight loss semaglutide produced over placebo, less than one percentage point was explained by gastrointestinal side effects. Feeling sick is not the mechanism. If you are nauseated and not losing weight, the nausea is not doing you any favours, and if you are losing weight with no side effects at all, nothing is wrong.
What to track on a GLP-1 — and why it changes the conversation
Almost every decision in the timeline above — delay the next step or not, adjust a diabetes medication or not, investigate a symptom or wait — is a prescriber's decision that gets made on the quality of what you can tell them. Memory produces "the nausea has been bad". A log produces "nausea 6/10 on days 1–3 after each injection, 2/10 by day 5, worse after the 1 mg step than the 0.5 mg step, always worse when I skip breakfast". Five streams are enough:
- Injection day and dose — the anchor every other line is read against.
- Side-effect severity per day — nausea, bowel pattern, fatigue, dizziness, each on a simple 0–10.
- Food — what and how much. Fatty, fried and very sweet foods are the usual triggers; our guide to GLP-1 food aversions covers what the changes mean.
- Water intake — because dehydration is the line between a bad week and a kidney problem.
- Weight as a 7-day rolling average, and mood — so the payoff and the cost sit on the same page.
tr8ck's GLP-1 module is built around exactly this: dose and injection-day logging, a side-effect tracker that scores severity per day and lines it up against your titration steps, plus food, water, weight and mood in one timeline — so when your prescriber asks how the last four weeks went, you can show them the wave instead of describing it.
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See the wave, not just the bad days
tr8ck's GLP-1 module logs dose, injection day, side-effect severity, food, water, weight and mood together — so you can show your prescriber exactly how each titration step went.
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Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. GLP-1 medications are prescription drugs. Never start, stop, or change the dose of any medication on your own — discuss it with your prescriber. Adverse-event rates from clinical trials are population averages and individual responses vary substantially; seek urgent care for severe or persistent symptoms.
Sources
- Wilding JPH et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). N Engl J Med. 2021;384(11):989–1002. doi:10.1056/NEJMoa2032183
- Wharton S et al. Gastrointestinal tolerability of once-weekly semaglutide 2.4 mg in adults with overweight or obesity, and the relationship between gastrointestinal adverse events and weight loss (pooled STEP 1–3, STEP 4). Diabetes Obes Metab. 2022;24(1):94–105. doi:10.1111/dom.14551
- Jastreboff AM et al. Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1). N Engl J Med. 2022;387(3):205–216. doi:10.1056/NEJMoa2206038
- WEGOVY (semaglutide) injection — U.S. Prescribing Information, Novo Nordisk (DailyMed, revised 2026): §2.2 dosage escalation, §5 warnings and precautions, §6.1 Table 3 adverse reactions, hair loss, gallbladder disease, heart rate
- ZEPBOUND (tirzepatide) injection — U.S. Prescribing Information, Eli Lilly (DailyMed, revised 2026): §2.1 dose escalation, §5 warnings and precautions, §6.1 Table 1 adverse reactions, hair loss, gallbladder disease, hypoglycemia
- Malkud S. Telogen Effluvium: A Review. J Clin Diagn Res. 2015;9(9):WE01–WE03. doi:10.7860/JCDR/2015/15219.6492
- NIDDK — Prescription Medications to Treat Overweight & Obesity (U.S. National Institutes of Health)