Short answer: The most common side effects of GLP-1 weight-loss injections are nausea, constipation, diarrhea, heartburn (reflux), fatigue and mild headache. They usually stand out in the first weeks after starting the medication and on dose-increase days, and they ease as your body adjusts.
Key takeaways
- The most common symptom is nausea, which usually peaks on days 2–3 after the injection.
- Eating slowly, avoiding fatty foods and making portions smaller noticeably reduce nausea.
- To prevent constipation, at least 2–2.5 liters of water a day and a fiber-rich diet are recommended.
- In case of severe abdominal pain or vomiting that does not stop, see a doctor without delay.
The most common side effects of GLP-1 weight-loss injections involve the digestive system: nausea, vomiting, diarrhea, constipation, abdominal pain and loss of appetite. These usually stand out at the start of treatment and after each dose increase, and they lessen as your body adjusts. There are also rare but serious conditions; the warning list at the end of this page covers them.
This page is for general information only and does not replace medical advice. Always share any side effect you experience with your doctor; do not stop the medication or change the dose on your own.
Common side effects
The following are listed as common in the official product information for products such as Mounjaro, Ozempic, Wegovy, Saxenda and Trulicity:
- Nausea: the most frequently reported effect
- Vomiting
- Diarrhea
- Constipation
- Abdominal pain, bloating, indigestion
- Loss of appetite: part of how the medication works
- Fatigue
- Redness or itching at the injection site
Why does it happen?
These medications slow stomach emptying. Slower digestion works by prolonging the feeling of fullness; the same mechanism also causes nausea and bloating. That is why side effects stand out at the start of treatment and with each dose increase, and lessen as the body adapts.
How long does it last?
Digestive side effects are usually most noticeable in the first days and in the few days after a dose increase. They ease over time. If they persist, are severe or get in the way of daily life, talk to your doctor. Options such as slowing down the dose steps may be considered.
Serious but rare conditions
The following symptoms are rare, but when they appear they call for seeing a doctor without delay:
| Symptom | What it may point to |
|---|---|
| Severe, persistent abdominal pain that spreads to the back; persistent vomiting | Inflammation of the pancreas (pancreatitis) |
| Pain in the upper right abdomen, fever, yellowing of the eyes or skin | Gallbladder problems |
| Swelling in the neck, hoarseness, difficulty swallowing, shortness of breath | Thyroid-related findings that need evaluation |
| Shaking, cold sweats, sudden hunger, dizziness, confusion | Low blood sugar (hypoglycemia) |
| Swelling of the face, lips or tongue, widespread rash, difficulty breathing | Allergic reaction |
| Sudden change in vision | Evaluation related to diabetic retinopathy |
| Reduced urination, excessive thirst, weakness | Fluid loss from vomiting or diarrhea and effects on the kidneys |
This table is not a diagnostic tool; only a doctor decides what a symptom means. Its purpose is to make the difference between "expected" and "needs urgent attention" visible.
General tips for reducing side effects
The following are general recommendations found in official product information and in patient information from health authorities. Your personal plan belongs to your doctor.
- Make portions smaller, eat slowly and stop when you feel full.
- Fatty and very spicy foods can make nausea worse.
- Spread your fluids across the day; if you have vomiting or diarrhea, watch for fluid loss.
- For constipation, fiber and movement; if it continues, talk to your doctor.
- Get enough protein to preserve muscle mass.
- Do not speed up dose increases on your own; titration exists precisely to reduce side effects.
Why logging side effects helps
At a follow-up visit, answering "how was last month?" accurately from memory is hard. Which day, after which dose, how severe: these details can affect dosing decisions, and they are usually exactly what people do not remember.
Haftra logs side effects with their severity and date, and shows which day of the dose cycle they happened on. You can also see anonymous reports from people using the same medication. It is free and can be used without an account, and then your records stay on your phone. It does not diagnose and does not suggest doses.
iPhone, iPad and Apple Watch. Free.
Frequently asked questions
Are weight-loss injections harmful?
Like every effective medication, this group has known side effects and risks. The lists above summarize them. The balance of benefit and risk differs from person to person, and only a doctor can make that assessment. With products obtained without a prescription, the risk is on a completely different level: counterfeit products of unknown content can cause serious harm.
When does the nausea from weight-loss injections go away?
Nausea usually lessens within the first 2–4 weeks as the body adapts to the active ingredient. It may return mildly for 2–3 days with each dose increase.
What can be done to reduce side effects?
Eating small, frequent meals, staying away from fatty and overly spicy foods, drinking plenty of water on injection day and injecting at night before bed can ease side effects.
Should I stop the medication if I get side effects?
Do not stop on your own. Most common digestive side effects are temporary and can be managed by adjusting the dose plan. For anything on the serious symptoms list, see a doctor without delay.
Do weight-loss injections cause fatigue?
Yes; temporary fatigue can occur in the first weeks because of the rapid drop in calorie intake and metabolic adaptation. Adequate protein and electrolyte intake is important.
Does it cause muscle loss?
With rapid weight loss, part of the weight lost can come from lean tissue. That is why adequate protein intake and regular movement are frequent recommendations during treatment. Tracking your body composition helps you see what kind of weight you are losing.
Related pages
Sources
- EMA - Mounjaro (tirzepatide)
- EMA - Ozempic (semaglutide)
- EMA - Wegovy (semaglutide)
- EMA - Saxenda (liraglutide)
- NHS - Obesity treatment
- U.S. Food and Drug Administration (FDA)
Last reviewed: August 2, 2026
