Short answer: The most common side effects of weight-loss injections are nausea, constipation, diarrhoea, heartburn (reflux), tiredness and mild headache. They are usually most noticeable in the first weeks after starting the medicine and on dose-increase days, and they ease as your body gets used to it.
Key takeaways
- The most common symptom is nausea, which usually peaks on days 2–3 after the injection.
- Eating slowly, avoiding fatty foods and having smaller portions noticeably reduce nausea.
- To prevent constipation, at least 2–2.5 litres of water a day and a fibre-rich diet are recommended.
- If you have severe abdominal pain or vomiting that will not stop, contact a doctor without delay.
The most common side effects of GLP-1 weight-loss injections involve the digestive system: nausea, vomiting, diarrhoea, constipation, abdominal pain and loss of appetite. They are usually most noticeable 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 tell your GP or prescriber about any side effect you have; do not stop the medicine or change the dose on your own.
Common side effects
The following are classed as common in the official product information for products such as Mounjaro, Ozempic, Wegovy, Saxenda and Trulicity:
- Nausea: the most frequently reported effect
- Vomiting
- Diarrhoea
- Constipation
- Abdominal pain, bloating, indigestion
- Loss of appetite: part of how the medicine works
- Tiredness
- Redness or itching at the injection site
Why does it happen?
These medicines slow down stomach emptying. Slower digestion works by prolonging the feeling of fullness; the same mechanism also causes nausea and bloating. That is why side effects are most noticeable at the start of treatment and with each dose increase, and ease as the body adapts.
How long does it last?
Digestive side effects are mostly at their strongest in the first days and in the few days after a dose increase. They ease over time. If they carry on, are severe or get in the way of daily life, speak to your GP or prescriber. Options such as slowing down the dose steps may be considered.
Serious but rare conditions
The following symptoms are rare, but if they appear you need to contact a doctor without delay:
| Symptom | What it may point to |
|---|---|
| Severe abdominal pain that does not go away and 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 |
| A lump in the neck, hoarseness, difficulty swallowing, shortness of breath | Signs that need a thyroid-related assessment |
| Shaking, cold sweats, sudden hunger, dizziness, confusion | Low blood sugar (hypoglycaemia) |
| Swelling of the face, lips or tongue, widespread rash, difficulty breathing | Allergic reaction |
| A sudden change in vision | Assessment related to diabetic retinopathy |
| Passing less urine, extreme thirst, weakness | Fluid loss from vomiting or diarrhoea, and effects on the kidneys |
This table is not a diagnostic tool; only a doctor can decide 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 tips found in official product information and in patient information from health authorities. Your personal plan belongs to your GP or prescriber.
- Have smaller portions, eat slowly and stop when you feel full.
- Fatty and very spicy foods can make nausea worse.
- Spread your drinks across the day; if you are vomiting or have diarrhoea, watch out for fluid loss.
- For constipation, fibre and movement help; if it continues, speak to your GP or prescriber.
- 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 review appointment, it is hard to answer "how was last month?" accurately from memory. Which day, after which dose, how severe: these details can affect dosing decisions, and they are usually exactly the things people do not remember.
Haftra logs side effects with their severity and date, and shows which day of your dose cycle they happened on. You can also see anonymous reports from people using the same medicine. It is free and can be used without an account, and then your records stay on your phone. It does not make diagnoses or suggest doses.
iPhone, iPad and Apple Watch. Free.
Frequently asked questions
Are weight-loss injections harmful?
Like every effective medicine, this group has known side effects and risks. The lists above summarise them. The balance of benefit and risk varies 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: fake products with unknown contents can cause serious harm.
When does the nausea from weight-loss injections go away?
Nausea usually eases 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.
Do weight-loss injections cause tiredness?
Yes; because calorie intake drops quickly and the metabolism adapts, temporary tiredness can occur in the first weeks. Getting enough protein and electrolytes is important.
What can be done to reduce side effects?
Eating little and often, staying away from fatty and very spicy foods, drinking plenty of water on injection day and injecting at night before bed can ease side effects.
Should I stop the medicine 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 dosing plan. For anything on the serious symptom list, however, contact a doctor without delay.
Will I lose muscle?
With rapid weight loss, part of the weight lost can come from lean tissue. That is why enough protein and regular movement are among the most common 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
- MHRA (Medicines and Healthcare products Regulatory Agency)
Last reviewed: 2 August 2026
