If you are starting a weight-loss injection — or thinking about it — it is completely normal to feel a little anxious about side effects. The good news is that the most common ones are digestive, and they affect many people most in the early weeks.[1] They tend to be mild or moderate, and they usually ease as your body gets used to the medicine.[2] This guide explains what is common, what you can do at home, and when to contact us.
This is general information, not personal medical advice. It is not a recommendation to start any particular medicine. These are prescription-only medicines. We only supply them after a consultation with our pharmacist, who checks that the treatment is suitable for you — there is no automatic approval.[7] If anything here worries you, please ask us. That is what we are here for.
Why these medicines can cause side effects
These weight-loss injections belong to a group of medicines called GLP-1 medicines. GLP-1 is a natural hormone your gut releases after you eat, and these medicines copy the way it works. The two used most often in the UK are tirzepatide and semaglutide.[1]
They work mainly by slowing how quickly your stomach empties. They also gently reduce your appetite. This means you feel full sooner, and stay full for longer.[1]
Because they act on your stomach and gut, the most common side effects are digestive ones. That is also why they often settle as your body adjusts over a few weeks.[2]
What is common, and how long it lasts
It helps to know roughly what to expect, so you can put any symptom in perspective. The most common side effects are digestive: feeling sick, being sick, diarrhoea, and constipation.[1]
Many people get one or more of these, especially in the first few weeks. They also tend to come back for a short time just after each dose increase. For most people they are mild or moderate, and they ease over time.[2]
Some people also notice tiredness, indigestion or a little dizziness. The exact pattern can differ from one person to the next, and between the two medicines. The official information that comes with your medicine lists the full picture, and our pharmacist can talk it through with you.[3]
The reassuring part is the timing. The picture below shows when side effects tend to be at their strongest, and how they usually fade.
Most of these effects are temporary. If a side effect is making daily life hard, tell us. We may be able to slow down a dose increase, or suggest practical changes that help.[2]
Settling the most common feelings
Feeling sick (nausea)
Feeling sick is the side effect people ask about most. A few simple habits help a lot:
- Eat smaller meals more often, rather than three large ones.
- Eat slowly, and stop at the first sign of comfortable fullness. Pushing past it is what brings on most nausea.
- Go easy on rich, fatty, fried or very sweet food, which tends to sit heavily.
- Stick to plain, gentle foods on a queasy day — toast, crackers, rice, plain chicken, bananas, clear soup.
- Get some fresh air, and avoid lying down straight after eating.
Constipation
Eating less, and drinking less without realising, can slow your bowels. To help things along, build up fibre gently with vegetables, fruit, wholegrains and beans. Drink plenty of fluids, and keep moving with a daily walk.[4]
Diarrhoea
If your bowels loosen instead, the main job is to keep your fluids up, so you do not become dehydrated. Small, frequent sips work well.[8] If it is severe, or lasts more than a day or two, get in touch with us.
When to manage at home, and when to contact us
Almost all of the common side effects can be managed at home with the steps above. But a few symptoms are different. They are your signal to stop and get advice, rather than wait it out. The chart below is a simple way to tell them apart.
Lowering your chances of side effects
You cannot promise yourself a side-effect-free experience, but a few things genuinely help to keep them gentle:
- Increase the dose slowly, exactly as advised. The step-up schedule is designed to let your body adjust and to keep digestive side effects as gentle as possible.[2]
- Eat protein first, and keep portions sensible. Little and often beats large and rushed.
- Stay hydrated through the day.
- Keep gentle activity going, such as a daily walk, which also helps your bowels.
- Tell us early if something is not right. It is far easier to adjust a plan sooner than later.
Reporting side effects helps everyone
If you have a side effect from any medicine, you can report it directly through the MHRA Yellow Card scheme.[6] Reports from patients and pharmacies help keep medicines safe for everyone. You can do this yourself online, and you are always welcome to mention it to us too.
How we support you
With us, weight-loss treatment is never just a medicine handed over. By law, these are prescription-only medicines, so they can only be supplied after a proper clinical assessment.[7] That starts with a consultation with our pharmacist to check the treatment is suitable and safe for you. It then continues with regular follow-up, where managing side effects is part of the conversation. If something is bothering you between appointments, you do not need to wait — reach out, and we will help.
If you would like to understand the options first, you can read our overview of UK weight-loss medications, our guide to how tirzepatide and semaglutide differ, and our guide to eating well on treatment.
Sources
- NHS — Semaglutide: about, who can take it, and side effects
- NICE — Tirzepatide for managing overweight and obesity: patient safety and dose escalation
- Patient information leaflet (the official information that comes with the medicine) — tirzepatide
- NHS — Constipation: self-help and treatment
- NHS — Acute pancreatitis: symptoms and when to get help
- MHRA — Yellow Card scheme for reporting side effects
- General Pharmaceutical Council (GPhC) — Providing pharmacy services online: requirements for supplying medicines safely
- NHS — Diarrhoea and vomiting: drinking fluids, dehydration and when to get help
Common questions
Are side effects from weight-loss injections dangerous?
For most people they are mild or moderate and temporary, mainly digestive symptoms like feeling sick or a change in your bowels, and they tend to ease as your body adjusts. A small number of effects are serious and need urgent help, such as severe tummy pain spreading to your back, or signs of a severe allergic reaction. The triage chart in this article shows how to tell them apart.
How long do the side effects last?
They tend to be strongest in the first few weeks, and again for a short time just after each dose increase, then ease off as your body adjusts. If a side effect is not settling after a few weeks, or it is making daily life hard, contact us so we can review your plan.
What can I do at home to feel less sick?
Eat smaller meals more often, eat slowly and stop when comfortably full, avoid rich or fatty food, and stick to plain, gentle foods on a queasy day. Staying hydrated and getting some fresh air also help. If you cannot keep food or drink down, please get in touch.
Will I definitely get side effects?
Not everyone does, and many people only have mild ones. Increasing the dose slowly as advised, eating sensibly, staying hydrated and keeping active all help to keep any effects gentle. We cannot promise a side-effect-free experience, but we can support you to manage it.
When should I get urgent help?
Seek urgent help (999 or NHS 111) for severe tummy pain that spreads to your back, vomiting you cannot stop, being unable to keep fluids down, or signs of a severe allergic reaction such as swelling of the face, lips or throat, or trouble breathing. Do not try to manage these with diet.
Can I just stop the medicine if I get side effects?
If you have a serious symptom, stop and seek medical advice promptly. For milder effects, it is best to talk to us first rather than stopping on your own, as we may be able to adjust your dose or suggest simple changes that help.
How do I report a side effect?
You can report any side effect from a medicine through the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk. You can do this yourself online, and you are welcome to mention it to us as well.
Do you check that the treatment is safe for me?
Yes. These are prescription-only medicines, so by law they can only be supplied after a proper clinical assessment. Our pharmacist reviews every request to check the treatment is suitable and safe for you, and there is no automatic approval. Managing side effects is part of your ongoing follow-up with us.
Thinking about weight-loss treatment?
Every request is reviewed by our prescribing pharmacist — there is no automatic approval. See the options and how the consultation works.
See weight-loss options →This article is general information and does not replace personal medical advice. Weight-loss injections are prescription-only medicines, supplied only after a clinical assessment by our pharmacist, with no automatic approval. Always consult a qualified healthcare professional before starting or changing any treatment, and do not stop a prescribed medicine without advice. If you experience side effects from a medicine, report them via the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk.