Your first week on a GLP-1: what to eat while your body adjusts

The 30-second summary

• Week one is the lowest dose, and its job is to let your digestion adjust. Stomach side effects are most common while the dose is being increased.
• Eat small and plain, with protein first. Smaller, more frequent meals, and going easy on fatty and high-fibre food for the first few days, can help.
• Drink steadily and know the red flags. Sickness and diarrhoea cost fluid, and severe stomach or back pain that does not go away needs a doctor.

What the first week actually is

Every weekly GLP-1 injection starts low and climbs.

• Wegovy starts at 0.25 mg for weeks 1 to 4. Its prescribing information says the dose is increased over 16 weeks "to reduce the likelihood of gastrointestinal symptoms".
• Ozempic also starts at 0.25 mg, and its prescribing information notes that this is not a maintenance dose.
• Mounjaro starts at 2.5 mg for 4 weeks, then goes up to 5 mg.

The side effects are mostly digestive. For Wegovy, the prescribing information says nausea, diarrhoea and vomiting were reported most often during dose increases, and that most were mild to moderate and did not last long. For Mounjaro, it says nausea, diarrhoea and vomiting happened mainly while the dose was being increased and became less common over time.

So the first week is not a test of willpower, and it is not the week to judge the medicine by. It is the week to eat in a way your stomach will put up with.

Before your first injection: stock up

The worst time to shop is the day you feel sick. A few things in the fridge and cupboard make the first week much easier:

• Protein that needs no cooking: eggs, Greek yoghurt, cottage cheese, tinned fish, tofu.
• Soup, homemade and frozen in single portions or bought ready to heat.
• Plain starch: bread for toast, rice, potatoes.
• Soft fruit and cooked vegetables.
• Things to sip: water, and ginger or peppermint tea, which the advisory mentions as helpful.

The joint advisory also notes that smoothies, cottage cheese and soup are often more appealing than heavier foods such as red meat, cold cuts or hard cheese when appetite is low. (See the GLP-1 grocery list.)

The first few days: small and plain

The 2025 joint advisory from the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association and The Obesity Society gives practical advice for this stage:

• Eat smaller meals, more often. Vomiting is more likely after a large meal.
• Go easy on fatty and high-fibre foods for the first few days. The advisory notes that fat and fibre both slow the stomach further.
• Do not go long without eating. Nausea often comes in the morning or after a long gap. If you get caught in a cycle of not eating because you feel sick, the advisory suggests eating breakfast and then a small meal every 3 to 4 hours, with enough to drink.
• Stop when you are full. On these medicines, full arrives sooner than you are used to. It is the medicine working.

Protein first, even on a small plate

When you can only manage a few bites, make them count. The advisory suggests eating protein-rich foods first in a meal, so they get eaten before fullness arrives. Two eggs, a pot of Greek yoghurt or a bowl of chicken soup will do more for you than a plate you cannot finish. (See protein on a GLP-1.)

Drink steadily through the day

Nausea, vomiting and diarrhoea all cost fluid. The prescribing information for both semaglutide and tirzepatide warns that these side effects can lead to dehydration, and advises taking care to avoid it. As a general guide, the NHS suggests 6 to 8 cups or glasses of fluid a day. (See hydration on a GLP-1.)

Bring fibre back in gently

Constipation is also common on these medicines, and the advisory encourages enough fluid and fibre from food to prevent it. The advisory's point is about timing: in the first few days, while your stomach is adjusting, lighter food may sit better. As things settle, add beans, lentils, oats, wholegrains and vegetables back a little at a time, with plenty to drink. (See fibre on a GLP-1 and constipation on a GLP-1.)

Three things to know from day one

Alcohol. The NHS says it is best to avoid alcohol while using semaglutide or tirzepatide, because it can make feeling or being sick worse. (See GLP-1 medicines and alcohol.)

The contraceptive pill on Mounjaro. Mounjaro's prescribing information says reduced effectiveness of oral contraceptives cannot be excluded. It advises switching to a non-oral method, or adding a barrier method such as condoms, for 4 weeks after starting and for 4 weeks after each dose increase. (See GLP-1 medicines and birth control.)

Insulin or a sulfonylurea. If you take either for diabetes, the prescribing information says your prescriber may need to lower that dose, because the combination can cause low blood sugar. Do not change it yourself. Make sure your prescriber knows before your first injection.

Symptoms that are not a first-week nuisance

Most first-week side effects are unpleasant rather than dangerous. A few are not. The NHS lists these among the rare but serious side effects:

• Severe pain in your stomach or back that does not go away, which can be a sign of an inflamed pancreas.
• Severe tummy pain from gallstones or an inflamed gallbladder, listed for tirzepatide.
• Signs of low blood sugar, such as sweating, shaking or feeling dizzy, which are more likely if you take other diabetes medicines.

The NHS advice is to call NHS 111 if you think you might be having a serious side effect. Contact your prescriber too if you cannot keep fluids down.

A gentle first-week day

This is an illustration, not a prescription. Each dish is a full recipe with its portion stated and its nutrition calculated, and each is below the collection's median for both fat and fibre:

• Breakfast: Quark dumplings with warm peaches
• Lunch: Chicken, lemon and rice soup
• Dinner: Lemon-oregano chicken with courgette orzo

If a whole portion is too much, eat half and keep the rest. Every recipe page lets you switch to a single serving. For more, browse the GLP-1 soup recipes.

What to do with this

Week one is about getting through comfortably. From week two, it helps to know your own numbers: the calculator works out your calorie target, your protein floor and how much you are likely to manage at your dose. If the days straight after your injection are the hard ones, read what to eat on injection day.

Get your numbers

Sources

1. Wegovy 2.4 mg FlexTouch solution for injection in pre-filled pen: Summary of Product Characteristics. electronic medicines compendium. emc
2. Ozempic 0.25 mg solution for injection in pre-filled pen: Summary of Product Characteristics. electronic medicines compendium. emc
3. Mounjaro KwikPen solution for injection in pre-filled pen: Summary of Product Characteristics. electronic medicines compendium. emc
4. Mozaffarian D, et al. Nutritional priorities to support GLP-1 therapy for obesity: a joint advisory from the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association, and The Obesity Society. Obesity Pillars 2025. PubMed
5. NHS. Semaglutide. nhs.uk
6. NHS. Tirzepatide. nhs.uk
7. NHS. The Eatwell Guide. nhs.uk

Medical disclaimer: Articles in the foodose research library are educational, not medical advice. Talk to your prescriber or a dietitian about your own nutrition. See our full medical disclaimer.