Your First Week on Mounjaro or Ozempic: What to Expect and What to Eat

Your first week on a GLP-1 is mostly about letting your body adjust. You start on a low dose designed for exactly that, side effects that do show up are usually mild and short-lived, and weight loss builds gradually from here. What you eat this week is about staying comfortable: small, low-fat meals, protein first, and steady fluids.

What to expect

The dose. Your first dose isn't meant to do the heavy lifting. The US label for Zepbound says its starting dose is for starting treatment and isn't approved as a maintenance dose, and both the Zepbound and Wegovy labels set out a gradual increase in dose, to reduce stomach side effects. Mounjaro and Zepbound are the same medicine, tirzepatide; Ozempic and Wegovy are both semaglutide. When your dose goes up is your prescriber's call.

Stomach side effects. These are the most common. In the STEP trials of semaglutide 2.4 mg, 44% of people had nausea at some point, 30% diarrhea, 25% vomiting and 24% constipation. But 98% of those episodes were mild to moderate, and they came mostly during or shortly after a dose increase. Tirzepatide looks similar: its side effects were mostly digestive, mostly mild to moderate, and mostly while the dose was going up.

How long they last. An expert consensus on managing these side effects sums up the pattern. Nausea is most common in the first four to five weeks, and a bout usually passes within about eight days. Diarrhea tends to start in the first four weeks and lasts around three days. Constipation lasts longest, a median of 47 days, and feeling full can make you drink less, which doesn't help.

You don't need to feel sick for it to work. In the same STEP trials, people lost about as much weight whether or not they had stomach side effects.

Other changes. Feeling tired is a common side effect, and some people go off foods they usually enjoy, especially at the start and after a dose increase.

The scale. Don't judge the medication by the first weigh-in. Weight loss on these drugs builds over months and is fastest in the first six.

Before your first injection

A little preparation makes the first days easier.

• Stock easy protein: Greek yogurt, eggs, cottage cheese, soup, milk or a ready-made protein shake. The 2025 joint advisory from four US obesity and nutrition societies notes that smoothies, protein drinks, soups and cottage cheese often go down better than heavier foods when appetite is low.
• Know your protein target. The protein calculator works it out. This week, getting close is enough.
• Put water where you'll see it. Constipation is the side effect that lingers, and fluid helps. The water calculator gives you a daily target.
• Set reminders to eat. When you're never hungry, it's easy to forget, and the advisory suggests an alarm or reminder.
• Keep alcohol to a minimum. The advisory says it can make nausea and reflux worse. More in alcohol on a GLP-1.

What to eat in the first few days

The goal this week isn't a perfect diet. It's eating enough, comfortably.

• Small and often. The advisory recommends smaller, more frequent meals for early side effects. Stop as soon as you feel full: vomiting is more likely after large meals.
• Go easy on fatty and high-fibre food for the first few days. The advisory recommends exactly that for early symptoms. Then build fibre back up over the following week or two, with more water, since constipation tends to start in the first month.
• Protein first. Eat the protein on your plate before anything else, so it's done if you fill up early.
• Don't wait to feel hungry. Nausea often strikes in the morning or after long gaps without food, and not eating can make it worse. If that's happening, the advisory suggests breakfast, then a small meal every three to four hours, with enough to drink. How much to eat on a GLP-1 covers the numbers.
• Try ginger or peppermint tea if you feel queasy; the advisory lists both as helpful.

Some gentle, protein-first recipes from the site that suit this week:

• Egg White Veggie Frittata: light and low in fat
• High-Protein Greek Yogurt Bowl: cold, no cooking
• Chicken and White Bean Soup: easy to get down on a bad day
• Berry Protein Smoothie: for when chewing feels like too much
• Nature's Ozempic Gelatin: a high-protein panna cotta that slips down easily

More ideas for queasy days are in managing nausea.

The days after each injection

Levels of the medication peak about a day after a tirzepatide injection (Mounjaro, Zepbound) and one to three days after a semaglutide injection (Ozempic, Wegovy). Keep those days for the easiest food on your list, and save anything demanding, like a big meal out, for later in the week. Eating on injection day has more.

When to get help

Speak to your prescriber or a pharmacist if side effects bother you or don't go away. Get medical help urgently if you have:

• severe pain in your stomach or back that doesn't go away, which can be a sign of an inflamed pancreas
• severe tummy pain, which can also come from gallstones or an inflamed gallbladder
• vomiting or diarrhea that stops you keeping fluids down, since dehydration can damage your kidneys
• a swollen throat or tongue, a raised itchy rash or difficulty breathing, which are signs of an allergic reaction
• sweating, shaking, dizziness or confusion, which can mean low blood sugar, more likely if you also take insulin or a sulfonylurea

The NHS lists all the serious side effects on its semaglutide and tirzepatide pages. In the UK, NHS 111 can advise, and 999 is for emergencies; in the US, call your doctor, or 911 in an emergency.

Want the whole week planned?

The free 7-day starter at the bottom of this page lays out every meal for the week along these lines, with a protein cheat-sheet and a grocery list.

This is general nutrition information, not medical advice. Follow your prescriber's instructions for your medication.