GLP-1 side effects, month by month: what is normal, what is not.

1. Why side effects exist on GLP-1 therapy

GLP-1 receptor agonists (semaglutide, liraglutide) and dual GLP-1/GIP agonists (tirzepatide) work, in large part, by slowing gastric emptying and reducing appetite signals in the brain. Both mechanisms are exactly what makes the treatment effective, and both also explain most of the side effects. Nausea, early satiety, constipation and reflux are not random: they are the visible side of how the medication acts on your digestive system.

This matters for one reason: most side effects are not a sign that something is going wrong. They are a sign that the medication is doing what it was designed to do, in a body that is still adapting. The literature is consistent on this point. In the STEP-1 trial of weekly semaglutide 2.4 mg in adults living with overweight or obesity, gastrointestinal side effects were reported by 74.2% of participants on semaglutide vs 47.9% on placebo, but they were mostly mild to moderate, transient, and most pronounced during dose escalation.¹ SURMOUNT-1 (tirzepatide) showed a similar pattern.²

2. The first 4 weeks: orientation phase

The first month is usually the noisiest. The dose is being escalated, your body is calibrating, and your daily routine is finding its new shape. Most people on weekly semaglutide start at 0.25 mg, and most people on tirzepatide start at 2.5 mg. These starting doses are not therapeutic in themselves: they exist to let your body get used to the molecule.

Common during weeks 1 to 4:

• Nausea, often within the 24 to 48 hours after the injection. Frequently mild, sometimes moderate.
• Early satiety: you feel full faster, and meals you used to finish now feel too big.
• Mild fatigue: this is partly the digestive load, partly the lower energy intake.
• Headaches: usually related to insufficient hydration.
• Constipation: the most common bowel-side effect, often under-reported.

What helps in this phase:

• Smaller portions, more frequent. Five small meals can be more comfortable than three normal-sized ones.
• Protein and fibre at every meal. They counteract both nausea and constipation.
• Hydration. 1.5 to 2 L of water per day is a reasonable baseline. More on injection days.
• Plain, low-fat foods on the first 24 hours after the injection. Save the rich meals for the days when your stomach is calmest.
• Avoid alcohol on injection day. It worsens nausea and dehydration.

3. Weeks 4 to 12: the first dose escalations

Most protocols escalate the dose every 4 weeks, until the maintenance dose is reached. For semaglutide, that means 0.5 mg, 1 mg, 1.7 mg, 2.4 mg. For tirzepatide, 5 mg, 7.5 mg, 10 mg, 12.5 mg, 15 mg. Each dose step often brings back, briefly, the side effects of the start. This is normal. It is also why your physician will sometimes hold a dose for an extra month before going up.

What is new in this phase:

• Sulfur burps: a less-discussed but very common GLP-1 side effect. Slow gastric emptying lets fermentation happen further upstream.
• Reflux: lying down too soon after a meal becomes uncomfortable. Wait 60 to 90 minutes.
• Hair shedding: more visible in months 3 to 6, often related to the rapid energy deficit and not to the molecule itself. Usually reversible.
• Mood shifts: appetite is also a mood signal. The first months can feel emotionally flat for some people. This is real, and worth bringing up.

4. Months 3 to 6: the plateau of tolerance

By the third or fourth month, most people on a stable dose report a meaningful reduction in nausea and gastrointestinal symptoms. The body has adjusted; the routine is in place; meals have re-shaped. This is also when weight loss often becomes visible to others, and that, in turn, brings its own social and emotional load (more on this in our article on the mental load of being on an AOM).

Three things to watch in this phase:

• Lean mass loss: published estimates of lean-mass loss range from roughly 25% to 40% of total weight lost, with around 25% in the SURMOUNT-1 body-composition substudy on tirzepatide² and closer to 35-40% in the STEP-1 DXA substudy on semaglutide.¹ The French GCC-CSO/FORCE 2026 position paper cites approximately 25% as the typical value (Avis n°16). Resistance training and adequate protein intake are not optional. See our article on muscle, bone and GLP-1.
• Micronutrient intake: with smaller portions, vitamin and mineral intake drops. Vitamin B12 in particular has slow stores and a long warning curve. See our article on GLP-1 and vitamin B12.
• Hydration: a recurring contributor to fatigue and headaches in this period.

5. Months 6 to 12: the long routine

Once at maintenance dose, side effects are usually stable, predictable, and manageable. The challenge shifts: you now have to live this routine for an open-ended duration. Adherence becomes the central topic. Real-world data shows that around half of people on GLP-1 therapy for obesity discontinue within the first year, often without telling their physician. The reasons are not what people assume. We unpack this in why half of GLP-1 patients stop in the first year.

What can still appear or change in this phase:

• Plateau: weight stops moving for several weeks. Usually not a sign that the medication is failing. Read our article on plateau and rebound.
• Gallbladder pain: rapid weight loss is a known risk factor for gallstones. Right-upper-quadrant pain after a fatty meal warrants a medical consultation. The French GCC-CSO/FORCE 2026 position paper (partie V-3) notes that prophylactic ursodeoxycholic acid may be considered by the prescribing physician in defined cases of very rapid weight loss.
• Pancreatitis (rare): severe, persistent upper abdominal pain radiating to the back is a red flag. Stop the medication and seek urgent care. This guidance is consistent both with the Wegovy and Zepbound product labels (Warnings and Precautions, section 5.1) and with the GCC-CSO/FORCE 2026 Avis n°17.³⁵

6. Red flags: do not wait

Most side effects are tolerable, predictable, and time-limited. A small number are not. The following warrant a call to your physician or, if severe, urgent care:

• Severe, persistent abdominal pain, especially radiating to the back: rule out pancreatitis.
• Right-upper-quadrant pain after meals, jaundice, dark urine: rule out gallbladder disease.
• Vomiting that prevents you from drinking: dehydration risk, possible electrolyte issues.
• Signs of severe allergic reaction (swelling, breathing difficulty): emergency.
• Vision changes (especially if you have type 2 diabetes): bring up at the next consultation.
• Severe mood changes, persistent low mood, or suicidal thoughts: contact your physician immediately. The signal needs to be heard.
• Persistent dehydration, especially in older adults or in hot weather.

None of this is a reason to be afraid of the treatment. It is a reason to be informed. The clinical data on cardiovascular safety of GLP-1 therapy in obesity is strong. The SELECT trial showed a 20% reduction in major adverse cardiovascular events with semaglutide in adults with cardiovascular disease and overweight or obesity, without diabetes.⁴ The point is to recognise the rare events fast, not to live in fear of them.

7. What to bring to your next consultation

The most useful thing you can do for your physician is to bring structured information: when did the side effect start, how often, how severe, what made it better or worse, what dose were you on. This is harder than it sounds when you are in the middle of it, which is exactly why Boli Care exists.

If you do not use Boli yet, even a one-page handwritten summary helps. We have built a short checklist in our article on preparing your next AOM consultation.

References

1. Wilding JPH, Batterham RL, Calanna S, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity. New England Journal of Medicine 2021;384(11):989-1002. doi:10.1056/NEJMoa2032183. PMID 33567185.
2. Jastreboff AM, Aronne LJ, Ahmad NN, et al. Tirzepatide Once Weekly for the Treatment of Obesity. New England Journal of Medicine 2022;387(3):205-216. doi:10.1056/NEJMoa2206038. PMID 35658024.
3. Sodhi M, Rezaeianzadeh R, Kezouh A, Etminan M. Risk of Gastrointestinal Adverse Events Associated With Glucagon-Like Peptide-1 Receptor Agonists for Weight Loss. JAMA 2023;330(18):1795-1797. doi:10.1001/jama.2023.19574. PMID 37796527.
4. Lincoff AM, Brown-Frandsen K, Colhoun HM, et al. Semaglutide and Cardiovascular Outcomes in Obesity without Diabetes. New England Journal of Medicine 2023;389(24):2221-2232. doi:10.1056/NEJMoa2307563. PMID 37952131.
5. Novo Nordisk. Wegovy (semaglutide) injection - Highlights of Prescribing Information, section 5.1 (Risk of Acute Pancreatitis). U.S. Food and Drug Administration. accessdata.fda.gov. Aron-Wisnewsky J, Disse E, et al. Position paper du GCC-CSO/FORCE sur le traitement medicamenteux de l'obesite (TMO), Avis n°17 et partie V-3. Medecine des Maladies Metaboliques 2025/2026. doi:10.1016/j.mmm.2025.10.003.