The most common weight loss injection side effects are nausea, vomiting, diarrhoea, constipation and stomach discomfort. They tend to be strongest when you start or after a dose increase, and they often ease over time. Contact a pharmacist or prescriber urgently for severe stomach pain, persistent vomiting, signs of dehydration or an allergic reaction.

This guide covers what is normal, what helps and what is not normal. If you want the background first, read our pharmacist’s explainer on how weight loss injections work.

Most common weight loss injection side effects

Most people who take a weight loss jab notice at least one side effect, and most of them are digestive. That fits how GLP-1 side effects work, which the next section explains. Here is how they split into common, less common and urgent.

Common

• Nausea
• Vomiting
• Diarrhoea
• Constipation
• Indigestion, heartburn or reflux
• Stomach pain, bloating, wind or burping
• Tiredness or headache

Less common

• Dizziness, especially if you are eating or drinking much less than usual
• Hair thinning, often linked to losing weight quickly
• Changes in taste, or going off foods you used to enjoy
• Redness, itching or a small lump where you inject
• Gallstones
•  Low blood sugar, mainly if you also take insulin or certain diabetes tablets

Urgent

• Severe or lasting stomach pain, with or without vomiting
• Vomiting that will not stop, or not being able to keep fluids down
• Signs of dehydration, such as very dark urine, passing little urine, dizziness or a dry mouth
• Swelling of the face, lips or tongue, or trouble breathing
• Yellow skin or eyes, or fever with pain in the upper right of your tummy

The urgent list is covered in more detail further down.

How long do weight loss injection side effects last?

For most people, digestive side effects last from a few days to a few weeks. They usually show up in the first weeks of treatment and again after each dose increase, then settle as your body adjusts to that dose. Nausea is often worst in the days after an injection. Constipation can last longer and may need steady management. If side effects go on for more than a few weeks, stop you eating and drinking normally, or keep getting worse, speak to a pharmacist or your prescriber. The dose can be held or lowered.

Why side effects happen

Weight loss injections belong to a group of medicines that copy a natural gut hormone called GLP-1. They lower appetite and slow gastric emptying, which means food stays in your stomach for longer. That helps you feel full on less food, and it also explains most of the side effects.

• Slower gastric emptying: nausea, feeling over-full, reflux, bloating, and vomiting if you eat too much or too fast.
• Less food and fluid, plus slower gut movement: constipation.
• Changes in how the gut moves: diarrhoea or loose stools.
• Rapid weight loss: gallstones and, rarely, gallbladder problems.

Side effects are usually strongest at the start because your body has not adjusted yet. That is also why treatment begins on a low dose and steps up slowly. Each increase can bring a short return of symptoms. People respond differently, so a dose that suits a friend may not suit you.

We do not repeat the full mechanism here. Our post on how weight loss injections work goes through it step by step.

Are weight loss injections safe?

When they are prescribed after a proper assessment and monitored, weight loss injections are an established treatment, and most side effects can be managed. They are not right for everyone, which is why the safety checks below come before the first dose.

How to manage nausea, constipation, diarrhoea and reflux

Small changes to how you eat and drink fix a lot of problems. Start with these habits, then use the table for specific symptoms.

• Eat smaller meals, eat slowly and stop when you feel full.
• Go easy on greasy, fried, spicy and very sugary food, especially in the days after an injection.
• Sip fluids through the day. About 6 to 8 glasses is a good target.
• Limit alcohol and fizzy drinks.
• Do not skip meals completely. Aim for some protein at each one.

If this happens, try this

If this happens	Try this	Contact us if
Nausea	Eat small, plain meals slowly. Cold or room-temperature food smells less. Sip fluids. Fresh air and ginger can help.	You cannot keep fluids down for 24 hours, or it has not eased after a few weeks.
Vomiting	Take small, frequent sips of water or oral rehydration solution. Leave heavy food until you feel settled.	You vomit more than once or twice a day, it lasts over 24 hours, or you have severe stomach pain or signs of dehydration.
Constipation	Drink more fluid. Build fibre up slowly towards about 30g a day (oats, fruit, veg, pulses, wholegrain). Move gently every day. Do not ignore the urge to go.	Diet changes are not enough, or you have severe pain, worsening swelling, or cannot pass wind or stool.
Diarrhoea	Sip fluids often. Use oral rehydration sachets if needed. Keep food plain and cut back on fatty or very sugary food and sugar-free sweeteners.	It lasts more than 2 to 3 days, there is blood in the stool, or you feel dizzy or very thirsty.
Reflux or heartburn	Eat smaller meals. Finish eating 2 to 3 hours before bed. Avoid fatty, spicy and acidic food, coffee and alcohol. Raise the head of your bed.	Symptoms continue despite simple measures, or you have pain on swallowing, black stools or vomit with blood.
Bloating and wind	Eat slowly. Avoid fizzy drinks and chewing gum. A short walk after meals helps.	Swelling or pain keeps getting worse.

A pharmacist can suggest an over-the-counter option for constipation, reflux or diarrhoea. Ask first, because some products are not suitable alongside other medicines you take.

Serious side effects and when to get urgent help

Serious side effects are uncommon, but they need quick action. The main ones to know about are pancreatitis (inflammation of the pancreas), gallbladder problems, dehydration that strains the kidneys, and severe allergic reactions.

Call 999 or go to A&E

• Swelling of the face, lips, tongue or throat, trouble breathing or swallowing, or a rash that spreads fast. These can be signs of a severe allergic reaction.
• Severe stomach pain that does not ease, especially if it spreads to your back or comes with vomiting.
• Fainting, confusion or a racing heartbeat alongside heavy vomiting or diarrhoea.

Contact your prescriber, pharmacist or NHS 111 the same day

• Vomiting or diarrhoea that will not stop, or signs of dehydration.
• Pain in the upper right of your tummy, fever, or yellow skin or eyes. These can point to gallbladder problems.
• Constipation with severe pain or swelling, or being unable to pass wind or stool.
• Shakiness, sweating, confusion or a fast heartbeat if you take diabetes medicines. This can be low blood sugar.
• Changes in your eyesight, if you have diabetes.
•  A lump in your neck, ongoing hoarseness or trouble swallowing.

If you think you are having a serious reaction, do not inject again until a clinician has reviewed you. Not sure which service to use? Our guide on whether to see a GP or a pharmacist explains what each can help with. For anything severe, use 999, A&E or NHS 111 rather than waiting for an appointment.

Safety checks before and during treatment

Whether weight loss injections are safe for you depends on your history and the other medicines you take. A good consultation covers the checks below before any prescription is issued. If you are unsure whether treatment suits you at all, read the signs you may benefit from a medical weight loss programme.

Medical history

Tell the pharmacist or prescriber if you have, or have ever had:

• Pancreatitis or gallbladder disease
• Kidney or liver problems
• Thyroid problems, or a family history of certain thyroid cancers
•  Diabetes, and which medicines you take for it
• Eye problems linked to diabetes
• Stomach or bowel conditions, or bowel or stomach surgery
•  An eating disorder
• Heart conditions

Other medicines

These injections slow stomach emptying, which can change how some tablets are absorbed. Insulin and some diabetes tablets can raise the risk of low blood sugar, so doses may need adjusting. Medicines such as warfarin may need extra monitoring. Tell us about everything you take, including supplements and anything bought over the counter.

Only use medicines supplied by a registered pharmacy after a prescriber has assessed you. The MHRA has warned about fake weight loss pens sold online.

Contraception, pregnancy and surgery

• The contraceptive pill: with some weight loss injections, the pill can work less well, mainly after you start and after each dose increase. Prescribers usually advise adding a barrier method such as condoms for 4 weeks after starting and after each increase, or switching method. Which advice applies depends on your medicine, so check with your pharmacist. If you need to review or change your pill, our NHS contraception service can help.
• Pregnancy and breastfeeding: these injections are not used in pregnancy or while breastfeeding. If you are planning a pregnancy, tell us early, because treatment usually needs to stop well beforehand.
• Surgery or sedation: tell the surgical or anaesthetic team that you take a weight loss injection. Slower stomach emptying can raise the risk of food being in your stomach during anaesthesia.

You can read what the first appointment involves in what to expect at your first consultation.

What follow-up looks like at the clinic

Follow-up is where side effects get caught early. At our pharmacist weight loss clinic in Burnham, serving Slough and surrounding areas, it usually covers:

1. A check-in after you start. How you feel, how you are eating and drinking, your bowel habits and any side effects.
2. A review before each dose increase. Doses go up gradually, usually at least four weeks apart. We only step up if you are coping well. If side effects are strong, the dose can be held or lowered.
3. Measurements. Your weight, and other checks such as blood pressure where needed.
4.  A recheck of your medicines and contraception. Anything new since your last visit is worth mentioning.
5. Practical support. Advice on protein, fibre, fluids, activity and eating habits, alongside the treatment.
6. A plan if it is not working. If side effects are too much or the results are not what you hoped for, we talk through changing the plan or stopping.

Between visits, call the pharmacy on 01628 603 170 if a side effect worries you. You do not need to wait for your next review.

FAQs

How long do side effects last?

Usually a few days to a few weeks. They tend to appear when you start and after each dose increase, then settle as your body adjusts. Constipation can last longer. If side effects continue for more than a few weeks or stop you eating and drinking, speak to a pharmacist or prescriber.

Can I eat normally on weight loss injections?

Mostly yes, but you will probably feel full sooner. Eat smaller portions, eat slowly and stop when you are full. Fatty, fried and very sugary food is the most common trigger for nausea and reflux. Keep drinking fluids and include protein at meals so you do not under-eat.

Can weight loss injections affect the contraceptive pill?

With some injections, yes. The pill can be less effective after you start and after each dose increase, so extra protection such as condoms is usually advised for 4 weeks each time. Severe vomiting or diarrhoea can also stop the pill working. Ask your pharmacist what applies to your medicine and pill.

What should I do if I miss a dose?

The rules depend on the medicine, so check your patient leaflet and ask us. In general, weekly injections can be taken once you remember if it is within a few days of the missed dose. After that, skip it and take the next one on your usual day. Never double up to catch up. If you have missed more than one dose, speak to your prescriber before restarting, as you may need to go back to a lower dose.

Who should not use weight loss injections?

They are generally not suitable if you are pregnant, breastfeeding or planning a pregnancy, are allergic to the medicine or its ingredients, have a personal or family history of certain thyroid cancers, or have an eating disorder. Caution is needed with a history of pancreatitis, gallbladder disease or serious stomach and bowel conditions. A prescriber decides after reviewing your history, so this list is not complete.

Talk it through at a private consultation

Whether you are thinking about treatment or already taking it and side effects are getting in the way, you can book a private consultation at Burnham Health Pharmacy, 30 High Street, Burnham SL1 7JP. We serve Slough and surrounding areas. We will talk through suitability, safety checks and follow-up, and answer your questions.