Important Note This article is for educational purposes only. It does not constitute medical advice. Semaglutide is a prescription medicine and should only be used under the supervision of a qualified healthcare professional.
Semaglutide is generally well tolerated by most people, but like all medicines, it can cause side effects. The majority of side effects observed in clinical trials were related to the digestive system. Understanding these effects helps set realistic expectations for anyone considering or currently using the medication.
This page summarises the side effects reported in the major clinical trials of Semaglutide.
Most Common Side Effects
The most frequently reported side effects in both the STEP (weight management) and SUSTAIN (type 2 diabetes) trials were gastrointestinal. These included:
- Nausea
- Vomiting
- Diarrhoea
- Constipation
- Abdominal pain or discomfort
- Decreased appetite
These side effects were usually mild to moderate in severity and occurred most often during the first few weeks of treatment or when the dose was increased.
How Common Were These Side Effects?
In the STEP 1 trial (which studied semaglutide for weight management), gastrointestinal side effects were reported by a significant number of participants:
| Side Effect | Semaglutide Group | Placebo Group |
|---|---|---|
| Nausea | ~44% | ~17% |
| Diarrhoea | ~30% | ~16% |
| Vomiting | ~24% | ~6% |
| Constipation | ~24% | ~10% |
Most of these effects were temporary and tended to improve over time as the body adjusted to the medication. However, they were the main reason some participants reduced their dose or stopped treatment.
Factors That Influence Side Effects
Several factors can affect how likely someone is to experience side effects:
- Dose escalation speed: Faster increases are associated with more side effects.
- Individual sensitivity: Some people are more prone to gastrointestinal discomfort.
- Diet: Eating large or high-fat meals can worsen nausea and discomfort.
- Starting dose: Beginning at a lower dose and increasing gradually helps reduce side effects for many people.
In clinical trials, a gradual dose escalation schedule was used specifically to improve tolerability.
Other Reported Side Effects
In addition to gastrointestinal effects, other side effects observed in trials included:
- Fatigue
- Headache
- Dizziness
- Injection site reactions (redness, itching, or swelling)
These were generally less common than digestive side effects.
Serious Side Effects
Although rare, some serious side effects were reported during clinical trials:
- Pancreatitis (inflammation of the pancreas)
- Gallbladder problems (including gallstones)
- Severe allergic reactions (very rare)
- Thyroid tumours (observed in animal studies; human risk is still being studied)
Because of these potential risks, semaglutide is not recommended for people with a personal or family history of medullary thyroid carcinoma or Multiple Endocrine Neoplasia syndrome type 2.
Comparison with Tirzepatide
When comparing side effect profiles:
| Aspect | Semaglutide | Tirzepatide |
|---|---|---|
| Common side effects | Mainly gastrointestinal | Mainly gastrointestinal |
| Frequency of nausea | Moderate | Slightly higher in some studies |
| Discontinuation due to side effects | Relatively low | Slightly higher at higher doses |
| Overall tolerability | Good with gradual dosing | Good with gradual dosing |
Both medicines have similar types of side effects, though individual tolerance can vary.
Managing Side Effects
In clinical practice and trials, the following strategies helped many people tolerate Semaglutide better:
- Starting at a low dose and increasing slowly
- Eating smaller, low-fat meals
- Staying well hydrated
- Avoiding lying down immediately after eating
- Taking the injection at the same time each week
If side effects become severe or persistent, dose adjustment or temporary pause in treatment may be considered under medical supervision.
Summary
The most common side effects of semaglutide are gastrointestinal in nature, particularly nausea, vomiting, diarrhoea, and constipation. These effects are usually mild to moderate, occur mainly during dose escalation, and often improve over time. Serious side effects are uncommon but can occur. Clinical trials showed that gradual dose increases and lifestyle adjustments can help improve tolerability. As with any medication, individual responses vary, and ongoing monitoring by a healthcare professional is important.
References
- ClinicalTrials.gov – STEP Programme (Semaglutide). NCT03548987 and related entries.
- Wilding, J.P.H. et al. (2021). Once-Weekly Semaglutide in Adults with Overweight or Obesity. New England Journal of Medicine.
- Rubino, D. et al. (2022). Effect of Weekly Subcutaneous Semaglutide vs Placebo on Weight Loss Maintenance in Adults With Overweight or Obesity. JAMA.
- Medicines and Healthcare products Regulatory Agency (MHRA) – Guidance on GLP-1 receptor agonists.
For more context, read our main guide on Semaglutide and our comparison Semaglutide vs Tirzepatide.
Last Updated: July 2026