Started a GLP-1 drug like Ozempic or Mounjaro? Here’s what you need to know about nutrition

Around half a million Australians use GLP-1 medications for diabetes – and when you add people who use it for obesity and weight-related conditions, the total is much higher.

GLP-1s – such as semaglutide (Ozempic or Wegovy) and tirzepatide (Mounjaro) – are very effective at managing diabetes, obesity and metabolic or weight-related health conditions.

But they can come with gastrointestinal and other side effects, including nutritional deficiencies.

If you’re taking a GLP-1, here’s what you need to know about possible side effects and how to get enough nutrients as your food intake decreases.

How do these medicines work?

When you eat, your body makes the appetite hormones GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide).

These hormones affect :

• how hungry and full you feel
• how quickly your food passes through your intestines
• insulin production
• blood sugar regulation
• metabolic rate and energy expenditure
• the brain’s reward centre, which impacts whether foods feel satisfying to eat, or not.

When our body produces these hormones, they only remain active for a few minutes.

But medications containing manufactured GLP-1 and GIP are modified to resist being broken down . This is why these medications, known as GLP-1 and GIP receptor agonists, only need to be injected weekly.

How much weight do users lose?

Compared to placebo (dummy) medications, average weight losses are about 6.24 kilograms higher in adults with type 2 diabetes or obesity who received semaglutide (1mg weekly), and up to 10.8kg greater for tirzepatide (15mg weekly).

The exact amount depends on your underlying health, the GLP-1 medication, dosage and how long treatment lasts.

In studies lasting more than one year, weight losses are reported as a percentage of starting body weight, and by medication type and dosage.

Compared to a placebo , people taking a 2.4mg weekly dose of semaglutide lost an average of 10.2% of their starting body weight. Those taking a 7.2mg dose lost an average of 12.6%.

For tirzepatide, body weight losses were 9.5% (for a 5mg dose), 12.4% (for a 10mg dose) and 14.8% (for a 15mg dose).

Users’ health and quality of life improve when weight loss is more than 5% of their body weight. But higher doses can be less tolerable, and come with more side effects .

This means medications should be tailored and balanced against side effects, health, wellbeing and individuals’ preferences.

What are the most common side effects?

Nausea was the most common side effect reported with GLP-1 use in a review covering more than 33,000 people, but depended on medication and dosage.

Another review of 48 studies found 21% of people reported nausea. Diarrhoea affected 10%, vomiting 9%, indigestion 9% and constipation 8%.

These rates might not sound high, but unpleasant side effects mean you’re more likely to stop taking the medication.

Then both body weight and weight-related risk factors, such as high blood sugar and cholesterol levels, start to return to baseline levels .

Other less common gut side effects include reflux, flatulence, burping, gallstone disease, abdominal distention, pain and discomfort.

In women, these side effects can reduce absorption of oral contraceptives, leading to more pregnancies.

Additional uncommon side effects include acute pancreatitis (inflammation of the pancreas), bowel obstruction, perioperative aspiration (where gastric acid enters the lungs), sarcopenia (muscle loss), retinopathies (eye damage), thyroid cancer, depression and suicidal thoughts.

Side effects should be discussed with and monitored by your doctor. They may recommend tips such as these, or other medications, to help manage common nutrition-related side effects.

How much less do people eat when taking GLP-1s?

My team reviewed 41 studies across 50,000 people taking GLP-1s. Surprisingly, only two had recorded data on what people ate in response to medication.

Some studies have used test meal experiments to measure acute effects on food intake.

In a review of four studies , adults were offered a buffet lunch under standardised conditions before they started GLP-1 medications and again after reaching stable doses. People were told to eat as much or as little as they wanted, while what they ate was covertly measured.

The average reduction in lunch energy intake was 1,132 kilojoules, about 25-33% less than baseline (pre-GLP-1) intakes.

A few studies applied buffet test meals to the entire day . They reported total kilojoule reductions ranging from 3,036kJ for semaglutide injections (1mg) to 3,581kJ for tirzepatide injections (15mg).

Such large reductions in total intake increase risk of nutrient deficiencies.

Malnutrition risk with GLP-1s

The risk of malnutrition increases if GLP-1 use triggers rapid weight loss. Specific nutrient deficiencies include:

• thiamine (B1) deficiency, where Wernicke’s encephalopathy can lead to irreversible brain damage
• metabolic acidosis , where the body accumulates too much acid, resulting from severe loss of appetite
• severely low plasma magnesium levels, which can affect your nerves, muscles and heart
• low blood sugar levels.

Deficiencies can happen to anyone. In 2025, pop singer Robbie Williams said he developed scurvy after taking medication to lose weight.

Signs of nutrient deficiencies include:

• fatigue
• irritability and mood changes
• poor appetite
• diarrhoea
• constipation
• anaemia
• cracks in the corners of your mouth
• dermatitis
• swollen tongue
• bleeding gums
• poor wound healing
• joint pain
• heart palpitations
• shortness of breath.

Consider keeping a food diary , either on paper or using an app, to help identify whether your nutrient intakes are adequate and your dietary intake is likely to be balanced.

Remind me, what’s a ‘balanced’ diet?

A balanced diet means you’re getting enough protein, fibre, vitamins and minerals.

The Australian Guide to Healthy Eating plate model helps guide meal building. Aim to fill:

• half your plate with vegetables or salad
• a quarter with lean protein such as chicken, beef, seafood, tofu or legumes
• a quarter with rice, pasta, potato or bread.

If you’re taking GLP-1 medications and experiencing appetite or gut side effects and your food intake is erratic, ask your doctor for a referral to a dietitian for help to improve nutrition-related health and wellbeing.

For more information about individual nutrients and recipe ideas to boost your intake, visit our No Money No Time website.

If this article has raised issues for you, or if you’re concerned about someone you know, call Lifeline on 13 11 14.