Weight loss medications, including Mounjaro, Wegovy (injection and tablets), and Foundayo, can cause a range of side effects while your body adjusts to treatment.

Nausea, vomiting, diarrhoea, constipation, sulphur burps, indigestion, headaches, skin sensitivity, and hair shedding are all known effects. They're most common during dose increases and tend to settle down within a few days to a couple of weeks. This guide explains why they happen and, most importantly, what you can do to manage each one.

If you experience any rarer, more serious side effects, stop taking your medication immediately and seek medical help.

Nausea (feeling sick) is one of the most common side effects when starting or increasing your dose. Vomiting (being sick) is less common but can happen too.

What can help:

• Eat smaller, more frequent meals (think three small meals and snacks in between, to prevent going long periods of time without food or drink) and stop eating as soon as you feel full

• Stick to bland, low-fat foods. Avoid hot, fried, greasy or strong-smelling snacks

• Sip water or herbal teas regularly, rather than drinking a lot in one go

• Try ginger, or foods containing ginger

• Avoid alcohol

• Eat slowly and stay upright for a while after eating

• Distract yourself with something you enjoy, or get some fresh air

A bland diet is easy to digest and can be useful to ease symptoms when used for short periods of time. Once symptoms ease, gradually reintroduce other foods. If discomfort comes back, return to bland foods briefly before trying again. Most people are able to return to a normal, balanced diet over time. Our free GLP-1 Nutrition Guide, developed by our Obesity Specialist Dietitian, can help with this transition (Phlo Clinic patients also have access to our Phlo Nourish Course, which can provide further support).

Weight loss medications can speed up how quickly food and fluids move through the gut, which can cause loose or watery stools.

What can help:

• Drink 2 to 3 litres of clear fluids a day (water, clear broth, sugar-free squash)

• Avoid artificial sweeteners. Sugar substitutes such as sorbitol, mannitol and xylitol, often found in sugar-free chewing gum and drinks, can worsen diarrhoea

• Eat small, frequent meals and chew thoroughly

• Choose low-fibre foods such as white bread, pasta, rice, plain biscuits, or cereals like cornflakes or rice krispies

• Oral rehydration solutions can help replace lost fluids and salts

Seek medical advice if diarrhoea is severe, lasts more than 48 hours, or comes with other symptoms.

These medications slow digestion down, which can make stools infrequent or hard, and leave your stomach feeling full or tight.

What can help:

• Drink 2 to 3 litres of water a day (herbal teas and sugar-free squash also count)

• Stay active. Walking or light exercise such as gentle yoga can help stimulate digestion

• Over-the-counter remedies such as Senna, Fybogel, or Bisacodyl/Dulcolax may help. Speak to your Phlo Pharmacist or GP before using these

• Add more fibre-rich foods to your diet, such as fruit, vegetables and wholegrains. Kiwi fruit can be especially helpful, as can flaxseeds, linseeds or chia seeds

• Limit ultra-processed foods. Foods such as biscuits, ice cream and crisps are often low in fibre and high in fat, which can contribute to constipation

These are burps that smell like rotten eggs, caused by gas released as food breaks down. They can be more noticeable on these medications because digestion is slower, so food stays in the stomach longer.

What can help:

• Limit sulphur-rich foods (eggs, broccoli, garlic)

• Eat slowly and chew well

• Avoid lying down straight after eating

• Stay well hydrated

Indigestion and reflux are well-documented effects of GLP-1 medications, including semaglutide (Wegovy), tirzepatide (Mounjaro), and orforglipron (Foundayo).

Why this happens: These medications slow how quickly food leaves your stomach, which contributes to feeling fuller for longer. Because food stays in the stomach longer, acid is more likely to back up into your food pipe (oesophagus), particularly after large or fatty meals, or if you lie down soon after eating. The exact reason isn't fully settled, and for most people symptoms are mild and settle as your body adjusts to the dose.

Common related symptoms: heartburn, bloating, belching, nausea, a feeling of fullness or food “sitting” in the stomach, and occasionally regurgitation.

What tends to make it worse:

• Large meals (since digestion is already slowed)

• High-fat, fried or spicy foods

• Lying down soon after eating

• Dose increases. Symptoms often flare after each step up, then settle within days to a couple of weeks

What can help:

• Smaller, more frequent meals rather than large ones

• Avoid lying down for 2 to 3 hours after eating

• Limit high-fat, fried, spicy food and alcohol

• Raise the head of your bed

• Eat slowly and stop before you feel overly full

Over-the-counter and prescribed treatment options, from fastest and shortest-acting to longest-lasting:

1. Antacids and alginates, such as Gaviscon Original/Advance and Rennie, neutralise acid directly. Gaviscon also forms a protective barrier over your stomach contents, which is particularly useful given how GLP-1 medications work. These are good for occasional, mild symptoms and fast relief (within minutes), but wear off within an hour or two.

2. H2 blockers, such as famotidine (usually prescription-only in the UK), reduce acid production. These are useful if symptoms occur a few times a week, and can be taken before a meal you expect to trigger symptoms, or at bedtime. They work within 30 to 60 minutes and last several hours.

3. Proton pump inhibitors (PPIs), such as omeprazole, lansoprazole and esomeprazole (omeprazole and esomeprazole are available over the counter in UK pharmacies), are best for frequent or persistent symptoms. They work best taken regularly, 30 to 60 minutes before your first meal of the day, rather than on demand. They can take 1 to 4 days to reach full effect, and long-term use should be reviewed with a clinician.

When to speak to a doctor: if reflux is persistent or worsening despite the steps above, isn't settling after a dose increase, or if you have difficulty or pain swallowing, food sticking on the way down, or frequent regurgitation. Chest pain should always be assessed by a medical professional. Never assume it's reflux.

Headaches can happen with these medications due to changes in eating patterns, dehydration, or low blood sugar.

What can help:

• Eat regularly (three well-balanced meals in addition to snacks) and drink enough fluids (your urine should be pale yellow or clear)

• Spend time in a quiet, calm space

• Use over-the-counter pain relief such as paracetamol

• Try relaxation techniques such as meditation

If headaches persist, contact our clinical team for advice.

This is a recognised side effect of GLP-1 medications, including Mounjaro, Wegovy (injection and tablets), and Foundayo, regardless of how you take them. It presents as unusual skin sensations such as burning, tingling, pins-and-needles, or increased sensitivity to touch.

It's usually mild to moderate, tends to happen more during dose increases, and appears to be dose-dependent. Based on current evidence, it doesn't appear to be harmful, and it typically resolves on its own after a few weeks at the same dose while your body adjusts, or with a dose reduction, or after stopping the medication if needed.

What can help:

• Avoid tight clothing

• Maintain adequate hydration and nutrition

Stop your medication and contact the Phlo team immediately if you experience swelling, blistering, numbness, or weakness.

Serious skin reactions (rare):

• Hypersensitivity reactions, including hives and angioedema (swelling of the deeper layers of skin, usually around the eyes and lips)

• Anaphylaxis. A severe allergic reaction involving hives, difficulty breathing, and swelling of the face or throat. This is a medical emergency. Seek immediate medical attention if you experience any of these symptoms.

If you're taking an oral medication (Wegovy tablets or Foundayo), this section doesn't apply to you.

Some people notice mild reactions at the injection site, such as redness, itching, swelling, or discomfort. A few simple habits can reduce this:

• Rotate your injection sites. Use different areas of the body, such as the top of the thigh or stomach, each time. See our advice hub for guidance.

• Let Mounjaro reach room temperature before injecting. Injecting it cold, straight from the fridge, can be more painful.

• Prepare the site properly. Clean the area with an alcohol wipe as instructed in your medication kit, and always use a new, unbent, sterile needle for each injection.

• Use ice to reduce swelling or muscle pain. Wrap crushed or cubed ice in a towel or plastic bag before applying. Never apply ice directly to the skin.

• Antihistamines, drowsy or non-drowsy, can help with itching around the body. Speak to your local pharmacist for advice on which is suitable for you.

• Wear loose, smooth clothing around the injection site to reduce friction and irritation

• Moisturise regularly and avoid harsh soaps or fragranced products if you have sensitive skin

You shouldn't experience severe pain from an injection. If you do, speak to your doctor as soon as possible. As with any side effect, monitor your symptoms and get in touch if they continue or get worse.

Losing weight quickly, whether through medication (Mounjaro, Wegovy, or Foundayo), diet, or exercise, can sometimes lead to a condition called telogen effluvium, or hair shedding.

Why it happens: Hair naturally cycles through three stages: growth, transition, and rest. The resting stage typically lasts around three months before that hair sheds naturally. Rapid weight loss can put the body under metabolic stress, causing a larger-than-usual proportion of hairs (around 7 to 35%, instead of the usual smaller percentage) to enter the resting or shedding phase at the same time. This can mean shedding around 300 hairs a day, compared with a typical baseline of 50 to 150 hairs. You might notice thinning at the hairline, a thinner ponytail, or more hair than usual in the shower or on your brush.

This is a recognised, temporary effect, and it isn't unique to weight loss medication. It can also be triggered by illness, physical trauma, childbirth, or psychological stress.

How to reduce the risk:

• Aim for gradual weight loss of around 1 to 2 lbs (0.5 to 1kg) per week. The risk and severity of hair shedding is linked to how quickly you lose weight

• Stick to your prescribed dose and escalation schedule, rather than progressing through doses too quickly. Please don't adjust your dose on your own. Speak to your prescriber first, so your rate of weight loss stays within a safe range

• Support your nutrition while on treatment:

– At least 1,200 to 1,500 kcal a day for women, and 1,500 to 1,800 kcal a day for men (this is the absolute minimum; most people will require more than this)

– Approximately 80 to 120g of protein a day

– Adequate healthy fats and iron. Low levels of either can independently contribute to hair thinning

Outlook: Hair shedding is generally self-limiting. Once your weight stabilises and your nutrition is on track, hair thickness typically returns to normal over time.

Next steps: If you're concerned about hair shedding, contact our Patient Care team. We can review your dose and nutritional intake, offer further guidance, or refer you to a specialist if needed. You can also read more on our website: Does Wegovy Cause Hair Loss and How to Treat It.

• If side effects start or worsen after a dose increase, consider discussing a slower titration with our Phlo Pharmacist rather than adjusting your dose yourself

• If symptoms persist or are significantly affecting your daily life, contact our Patient Care team or call 111

• Most side effects improve over time as your body adjusts. Support is always available if you need it

Track how you're feeling: if you'd find it helpful to track your side effects over time, BrightTrack may be worth exploring. Developed by the team behind Phlo Clinic, it's a free, standalone GLP-1 companion app, separate from your Phlo account, that lets you log symptoms, monitor patterns, and access expert guidance. Learn more about BrightTrack here.

Further reading:

• Bland Diet Guide | Phlo Clinic

• Mounjaro Side Effects Guide | Phlo Clinic

• Common Wegovy Side Effects Guide | Phlo Clinic

If you have any questions or concerns about side effects at any point during your treatment, our Patient Care team is here to help.