GLP-1 side effects: the numbers and what to do
Efectos secundarios del GLP-1: los números y qué hacer
Illustrative image. Does not represent guaranteed results.
In STEP 1, the New England Journal of Medicine trial with 1,961 adults, nausea and diarrhea were the most frequent adverse events with semaglutide. The authors described them as typically transient, mild to moderate, and noted they subsided over time.
"Transient and mild to moderate" is a scientific-paper phrase. In real life it can mean two uncomfortable weeks. This article is about those two weeks, and about the difference between what you ride out and what you report.
Key points
- In STEP 1, nausea and diarrhea were the most common adverse events with semaglutide, described as typically transient and mild to moderate.
- 4.5% of semaglutide participants discontinued due to gastrointestinal events, versus 0.8% on placebo.
- In SURMOUNT-1 with tirzepatide, discontinuation for adverse events was 4.3%, 7.1% and 6.2% by dose, versus 2.6% on placebo.
- In both trials, events clustered during dose-escalation periods.
- Severe persistent abdominal pain, or vomiting that prevents keeping fluids down, is not "part of the process" — report it.
In this article
Why they happen
It is not an allergic reaction or a sign something went wrong. It is the mechanism doing what it does.
GLP-1s slow gastric emptying. Food stays longer. That produces fullness — the intended effect — and it can also produce nausea, bloating and constipation, which are the other side of the same coin.
Why it worsens when the dose goes up
Because the effect on gastric emptying is proportional. That is why the trial protocols escalated the dose over 16 to 20 weeks instead of starting at full dose. Slow escalation is not bureaucracy: it is the strategy.
How common it actually was
The figure that best captures the size of the problem is not how many felt something, but how many had to stop.
| Trial | Discontinuation for adverse events |
|---|---|
| STEP 1 · semaglutide (gastrointestinal) | 4.5% |
| STEP 1 · placebo | 0.8% |
| SURMOUNT-1 · tirzepatide 5 mg | 4.3% |
| SURMOUNT-1 · tirzepatide 10 mg | 7.1% |
| SURMOUNT-1 · tirzepatide 15 mg | 6.2% |
| SURMOUNT-1 · placebo | 2.6% |
Read it both ways. Between 93% and 96% of participants did not stop for adverse events. And between 4% and 7% did. Both are true, and the second one matters if it is you.
Note also that placebo discontinuation was not zero. Some of what people feel starting any treatment happens without an active drug too.
What actually helps
None of this replaces the conversation with your clinician, and none of it is treatment. These are adjustments.
For nausea
Smaller, more frequent portions. Avoid fried food, very fatty meals and alcohol, especially in the days after a dose increase. Eat slowly and stop before feeling full.
For constipation
Water and fiber. When appetite drops, fluid and vegetable intake usually drop with it without you noticing. Daily movement helps too — we wrote about that in walking counts as exercise.
For fatigue
Often it is simply that you are eating considerably less than before. Checking that protein has not dropped too far is the first step: we cover it in protein and muscle during weight loss.
What resolves more cases than anything else
Adjusting the pace of dose escalation. That is the clinician's call, not yours, and it is a concrete reason to report symptoms instead of enduring them quietly.
What you do not tough out
Some symptoms are not part of the adjustment and require contacting your provider:
- Severe, persistent abdominal pain, especially radiating to the back.
- Vomiting that prevents keeping fluids down, or signs of dehydration.
- Pain in the upper right abdomen, yellowing of skin or eyes.
- Vision changes, if you have diabetes.
- A lump or swelling in the neck, persistent hoarseness or trouble swallowing.
The FDA maintains current public information on medications containing semaglutide, including safety warnings. It is linked in the sources.
Who is this NOT for?
If you already have warning symptoms from the list above, this article is not the place: call your provider.
If you have gastroparesis, prior pancreatitis or gallbladder disease, gastrointestinal effects stop being a manageable nuisance and become a serious clinical issue to evaluate before starting.
And if you are thinking of raising the dose on your own to speed things up, that is exactly where adverse events cluster in both trials. Slow escalation exists for that reason.
Frequently asked questions
How long does the nausea last?
In STEP 1 the authors described these events as typically transient and noted they subsided over time, clustering during dose escalation. We cannot give you an individual timeline — it varies from person to person.
If I have side effects, does that mean it is working?
No. There is no demonstrated relationship between the intensity of gastrointestinal effects and the outcome. Feeling less does not mean the medication is not acting.
Can I lower my own dose?
No. Reporting it to your clinician is the right path, and adjusting escalation pace is one of the most helpful tools. Doing it on your own breaks the follow-up.
Do anti-nausea medications help?
That is your provider's decision — they know your history and your other medications. It is not something to self-medicate.
What if I simply cannot tolerate it?
That happened to 4.5% of STEP 1 participants. It is a real possibility and not a personal failure. There are alternatives to evaluate with the clinician, and sometimes the honest conclusion is that this path is not yours.
Is constipation normal?
It is among the gastrointestinal effects reported with this class. Water, fiber and movement are the first things to check; if it persists, report it.
Do you offer support in Spanish?
Yes. All of our content and support is available in Spanish.
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
- Jastreboff AM, et al. Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1). N Engl J Med. 2022;387(3):205-216. — DOI
- FDA. Medications Containing Semaglutide Marketed for Type 2 Diabetes or Weight Loss. — U.S. Food and Drug Administration
- Wilding JPH, et al. Weight regain and cardiometabolic effects after withdrawal of semaglutide: the STEP 1 trial extension. Diabetes Obes Metab. 2022;24(8):1553-1564. — DOI
This article is informational and is not medical advice. SCTS1 does not prescribe medication. All medical care, prescriptions, and treatment plans are provided by licensed healthcare providers through our partner platform. Treatment eligibility is determined by a licensed provider. Compounded medications contain the same active ingredient class but are not FDA-approved finished pharmaceuticals. Individual results vary and are not guaranteed.