Weight-loss injection side effects: what is common and when to get help
Medically Reviewed by Dr. Pedro Yelamo Vera, MD
Licensed Physician No. 484814280 · Official Medical College of Bizkaia (Spain)Last reviewed: 12 October 2026
Weight loss injection side effects explained by a GP in Spain: common stomach effects, simple tips, rare serious risks, and when to call 112.
You don't have to figure this out alone
Dr. Pedro Yelamo Vera is an independent, bilingual GP licensed in Spain. Book a 15-minute video consultation in English or Spanish for €50, with a private electronic prescription issued under Spanish law when clinically appropriate.
Weight-loss injections, such as the GLP-1 medicines, often cause side effects. Most are mild stomach effects that settle with time. A few are rare but serious and need quick care. This guide explains what is common, what helps, and which signs mean you should get help. It is for expats, residents and travellers in Spain.
For a medical emergency, call 112 or go to the nearest local emergency department. Do not wait for a video consultation or an online reply.
- Nausea, vomiting, diarrhoea, constipation and stomach pain are the most common side effects.
- They are often worse at the start and after a dose increase. They tend to ease over time.
- Smaller meals, stopping when you feel full and enough fluids can help.
- Severe stomach pain, vomiting you cannot stop, signs of dehydration, sudden vision loss or a severe allergic reaction need urgent care.
- Tell your anaesthetist before any surgery or sedation.
Which medicines this covers
This guide covers GLP-1 medicines and the dual GIP/GLP-1 medicine. The main active ingredients in Spain are liraglutide (Saxenda), semaglutide (Wegovy, Ozempic) and tirzepatide (Mounjaro). They share most side effects because they work in a similar way. To learn how they work, read our guide on how GLP-1 weight-loss medicines work.
Always read the leaflet in your pack. It lists every known side effect for that product.
Common side effects
In the large trials, stomach and gut effects were the most common. Most were mild or moderate. They often came on in the first weeks or after a dose increase, and then eased.
Very common or common effects include:
- nausea and vomiting;
- diarrhoea or constipation;
- stomach pain, bloating, wind and burping;
- heartburn or acid reflux;
- headache, tiredness and dizziness;
- redness or itch where you inject;
- some hair loss, often when weight drops quickly;
- a change in how food tastes.
Some people stop treatment because of these effects. That is a valid choice. Talk to your doctor before you stop or change anything.
Simple tips for stomach side effects
These tips do not involve your dose. Never change your dose on your own. If side effects are hard to live with, talk to the doctor who prescribed the medicine.
- Eat smaller meals. Large meals sit in the stomach for longer and can bring on nausea.
- Stop when you feel full. Your fullness signal comes sooner now. Eating past it often leads to nausea or vomiting.
- Eat slowly. Put your fork down between bites.
- Go easy on rich, greasy or very sweet food. Many people find these make nausea worse.
- Drink enough fluids. Sip water through the day. Drink more if you have vomiting or diarrhoea.
- Help your bowels. Fluids, fibre and moving each day can ease constipation.
- Do not lie down straight after eating if you have reflux.
Eating less should never mean skipping food or fluids for long periods. If you have had an eating disorder, or eating feels hard to manage, tell your doctor.
Less common but serious side effects
Pancreatitis
Inflammation of the pancreas is uncommon but can be serious. The main sign is severe, constant pain in the upper belly. It often spreads to the back. You may also vomit. If this happens, stop the injection and get urgent care. If pancreatitis is confirmed, the medicine should not be restarted.
Gallbladder problems
Gallstones are a known side effect, and losing weight quickly adds to the risk. Watch for pain in the upper right belly, fever, or yellow skin or eyes. These need a prompt medical check.
Dehydration and kidney problems
Vomiting and diarrhoea can drain your body of fluid. In some people this harms the kidneys. Warning signs are passing very little urine, dark urine, a dry mouth, feeling faint or confusion. Older people and those with kidney disease are at more risk.
Low blood sugar
On their own, these medicines rarely cause low blood sugar. The risk rises if you also take insulin or a sulfonylurea. Signs include shaking, sweating, a racing heartbeat, hunger, confusion or feeling unwell. If you use these medicines, your doctor may need to adjust them. Ask how to check your blood sugar.
Surgery, endoscopy and sedation
These medicines slow down the emptying of the stomach. Food can stay in the stomach even after the usual fasting time. Under general anaesthesia or deep sedation, stomach contents could enter the lungs. This is called aspiration.
Tell the surgical team, your anaesthetist and your dentist that you use one of these medicines. Do this well before any planned procedure, including an endoscopy. Do not stop the medicine on your own. Ask the team for their plan.
Sudden vision loss
In 2025, the EMA concluded that NAION is a very rare side effect of semaglutide. NAION is damage to the nerve of the eye caused by poor blood flow. It may affect up to 1 in 10,000 people taking semaglutide. If you notice sudden loss of vision or eyesight that worsens quickly, get urgent care.
People with eye damage from diabetes (diabetic retinopathy) also need care. A quick drop in blood sugar can make it worse for a time. Keep up your eye checks.
Allergic reactions
Serious allergic reactions are rare. Swelling of the face, lips or throat, trouble breathing or a widespread rash need emergency care.
Contraception and pregnancy
These medicines are not used in pregnancy. If you could become pregnant, use reliable contraception during treatment. If you find you are pregnant, contact a doctor straight away.
The UK medicines regulator (MHRA) has advice for tirzepatide. In people who are overweight, it may make the contraceptive pill work less well. The MHRA advises adding a barrier method, such as condoms, or using a non-oral method. Do this for four weeks after starting, and for four weeks after each dose increase. Other options are a coil or implant. The EU product information takes a different view and says the pill needs no change. Ask your doctor which advice fits you.
Vomiting or severe diarrhoea can also stop the pill working, with any of these medicines. Follow the advice in your pill leaflet.
If you plan a pregnancy, the medicine must be stopped some weeks before you try. Your doctor will tell you how long.
Red flags: what to do
Use this table as a guide. If you are unsure, it is safer to seek care.
| Symptom | What to do |
|---|---|
| Swelling of the face, lips or throat, or trouble breathing | Call 112 |
| Severe, constant belly pain, often spreading to the back, with or without vomiting | Stop the injection and go to an emergency department, or call 112 |
| Signs of low blood sugar that do not improve, or confusion or fainting | Treat low blood sugar as advised; call 112 if it does not improve or the person is drowsy |
| Vomiting you cannot keep fluids down with, very little urine, dizziness when standing | Urgent in-person care: do not wait |
| Pain in the upper right belly, fever, or yellow skin or eyes | Urgent in-person care: do not wait |
| Sudden loss of vision or eyesight that worsens quickly | Urgent in-person care (emergency department) |
| A positive pregnancy test | Stop the injection and contact your doctor |
| Nausea, constipation or reflux that does not settle | Book a review with your doctor |
| Hair loss, low mood, or worry about how you are eating | Book a review with your doctor |
Fake pens are a real risk
Spain's medicines agency (AEMPS) has warned about illegal GLP-1 products sold online and on social media. They may contain the wrong amount of medicine, or harmful substances. In Spain, prescription medicines cannot be sold online. Use only a pharmacy. Read our guide to fake weight-loss pens in Spain.
How a review can help
Dr. Pedro Yelamo Vera offers a 30-minute Weight Optimization consultation by video for people in Spain. If you already use one of these medicines, the visit can look at your side effects, your other medicines, and your blood tests. It can also cover your plans for pregnancy or surgery. A video visit is not for emergencies, and it cannot replace a hands-on check.
Any change to treatment is a medical decision. Where clinically appropriate, following a genuine evaluation. Treatment and prescriptions are issued solely at the doctor's discretion based on evidence-based clinical guidelines. Any prescription is a private electronic prescription under Spanish law.
Dr. Pedro Yelamo Vera is a GP licensed in Spain (Colegiado No. 484814280).
Worried about side effects?
Book a video review with a GP in English or Spanish while you are in Spain.
FAQ
What are the most common weight loss injection side effects?
Nausea, vomiting, diarrhoea, constipation and stomach pain are the most common. They are often worse at the start and after a dose increase, and tend to ease.
How can I ease nausea on a GLP-1 medicine?
Eat smaller meals, eat slowly and stop when you feel full. Many people find rich or greasy food makes it worse. Sip fluids through the day. Do not change your dose on your own.
When is stomach pain an emergency?
Severe, constant pain in the upper belly, often spreading to the back, can be a sign of pancreatitis. Stop the injection and get urgent care, or call 112.
Do I need to tell my anaesthetist I use a weight-loss injection?
Yes. These medicines slow stomach emptying, which can raise the risk of aspiration under anaesthesia or deep sedation. Tell the team well before the procedure and do not stop the medicine on your own.
Does tirzepatide affect the contraceptive pill?
The UK regulator advises extra or non-oral contraception for four weeks after starting and after each dose increase. EU product information says no change is needed. Ask your doctor what fits you.
Can a video consultation help with side effects?
It can help review side effects, medicines and tests. It is not for emergencies. Where clinically appropriate, following a genuine evaluation.
Sources
- European Medicines Agency: Wegovy (semaglutide) product information
- European Medicines Agency: Mounjaro (tirzepatide) product information
- European Medicines Agency: PRAC concludes NAION is a very rare side effect of semaglutide (June 2025)
- MHRA: GLP-1 medicines for weight loss and diabetes, what you need to know
- AEMPS: EMA and HMA warn of illegal medicines sold as GLP-1 analogues (2025)
- Wilding JPH et al. Once-weekly semaglutide in adults with overweight or obesity. N Engl J Med 2021 (STEP 1)
- Jastreboff AM et al. Tirzepatide once weekly for the treatment of obesity. N Engl J Med 2022 (SURMOUNT-1)
Related guides
Medical Citations & Sources
This content is written and reviewed by a licensed physician with reference to the following official health authorities.
