What Does the Research Actually Say About Weight Loss Medications?
By Laekin Rose Accredited Practising Dietitian
What medications are available in Australia?
Oral medications
Orlistat (Xenical)
Phentermine
Naltrexone/Bupropion (Contrave)
Injectable medications
Liraglutide (GLP-1)
Semaglutide (GLP-1)
Tirzepatide (GLP-1 and GIP)
Who are weight loss medications intended for?
TGA approved guidelines, these medications can be prescribed when:
BMI > 30, or
BMI > 27 with at least one weight-related health condition
In combination with reduced calorie intake and increased physical activity
Oral medications
Typically work by reducing appetite, with expected weight loss of around 10–12% over 12 months.
Common side effects can include nausea, constipation, headache, dizziness, and dry mouth.
Orlistat works by blocking fat absorption, which can lead to gastrointestinal side effects. These can often be improved by adding fibre (e.g. psyllium) and monitoring fat intake. Long-term use may require supplementation of fat-soluble vitamins (A, D, E, K).
GLP-1 medications explained
GLP-1 medications have multi-factorial effects throughout the body:
Brain → increase satiety, reduce food noise
Stomach → slow gastric emptying (you feel full longer)
Pancreas → improve insulin production
Liver → reduce glucose production
Muscles → improve insulin sensitivity
Expected weight loss is 10 - 20% body weight loss over 12 - 18 months.
Common gastrointestinal symptoms include:
Nausea
Vomiting
Diarrhoea or constipation
Dyspepsia
These are usually temporary and can be reduced by:
Gradual dose increases
Smaller more frequent meals
Avoiding common dietary triggers (eg. fatty foods, spicy foods, alcohol)
Modifying fibre and fluid intake
What about weight loss supplements?
Most nutraceuticals (supplements) have little to no meaningful effect on weight loss, may cause side effects and are poorly regulated. A 2019 study found that over 50% of supplements were mislabelled, meaning what’s on the label isn’t always what you’re taking. Always discuss supplements with a qualified healthcare professional.
Muscle loss and weight loss medications
Weight loss isn’t just fat. With GLP-1 medications, up to 25% of weight loss may come from fat-free mass (FFM). Eating a high protein diet and including resistance-based exercise may help to preserve FFM. Monitoring body composition via regular DEXA scans is recommended.
Risk of malnutrition
Weight loss medications have significant impacts on appetite and nutrition status. People using weight loss medications may face an increased risk of malnutrition due to reduced food intake. Dietitians can help people to prevent malnutrition and ensure the best possible health outcomes whilst supporting individuals’ uniquenutrition needs and preferences.
What happens when you stop the medication?
Studies show that most people who stop taking weight loss medications will regain weight. Factors associated with long-term weight maintenance are:
Continued adherence to a reduced calorie intake
300–360 minutes of moderate intensity physical activity per week
Regular weigh-ins and monitoring of body composition
Ongoing professional support
The real impact
Weight loss medications can be highly effective tools when used appropriately. But they work best when combined with nutrition support, exercise and a commitment to lifelong behaviour change.
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