How Weight Loss Injections Work

The main injectable medicines for weight management belong to a class called GLP-1 receptor agonists or dual GIP/GLP-1 agonists. These medicines act on the body’s gut–brain axis, sending signals of fullness to the brain and slowing how quickly food leaves the stomach. Over time, this leads to smaller portions, fewer cravings, and lower calorie intake without the feeling of restriction common with dieting.

GLP-1 is a hormone naturally made in your gut after meals. It helps regulate blood sugar and makes you feel satisfied sooner.

After eating, GLP-1 is released from cells in the small intestine. It tells your pancreas to release insulin when glucose is high, suppresses glucagon (which raises blood sugar), and slows digestion.
GLP-1 injections copy this hormone’s effect, helping your body work more efficiently to manage appetite and blood sugar.

Weight loss injections help you feel full sooner and stay full longer. They also make your body use energy more efficiently. GLP-1 medicines act in three main ways:

• Brain: Reduce hunger signals and cravings.
• Stomach: Slow how fast food leaves the stomach.
• Pancreas: Improve insulin response and balance blood sugar.

This combined effect reduces calorie intake naturally while maintaining energy and satiety.

Wegovy

• Active ingredient: Semaglutide

• Dosing: Weekly

• Mechanism: GLP-1 agonist

• Main use: Weight management

Ozempic

• Active ingredient: Semaglutide

• Dosing: Weekly

• Mechanism: GLP-1 agonist

• Main use: Type 2 diabetes (sometimes prescribed off-label for weight management)

Saxenda (or generically as Nevolat)

• Active ingredient: Liraglutide

• Dosing: Daily

• Mechanism: GLP-1 agonist

• Main use: Weight management

Mounjaro

• Active ingredient: Tirzepatide

• Dosing: Weekly

• Mechanism: Dual GIP/GLP-1 agonist

• Main use: Weight and metabolic control

You may feel changes in appetite within the first few weeks. Visible weight change follows gradually over several months. Most people notice a reduced desire to snack or eat large portions after 2–4 weeks. Steady weight loss typically begins around month two and continues for up to a year.
Average clinical results show:

• Semaglutide (STEP 1): ~15 % mean weight loss.
• Tirzepatide (SURMOUNT-1): ~22 % mean weight loss.
• Liraglutide (SCALE): ~8 % mean weight loss.

Most side effects are mild and temporary. Nausea, constipation, and tiredness are the most common. Side effects usually occur when doses are increased too quickly. To help:

• Eat smaller, slower meals.
• Avoid large, high-fat meals during early weeks.
• Stay hydrated and include fibre and protein.
• Seek medical advice for persistent vomiting, severe stomach pain, or signs of dehydration.

Most weight loss injections are easy to use at home with a prefilled pen.

• Administer under the skin in an area where there is a larger fat layer such as abdomen, thigh, or upper arm.
• Rotate sites weekly.
• Keep pens refrigerated until first use, then at room temperature per manufacturer guidance.
• Dispose of needles safely in a sharps bin after every use.

The injection helps reduce hunger, but lasting results come from improved eating and movement habits. GLP-1 injections make it easier to:

• Eat smaller, higher-quality meals.
• Maintain consistent routines.
• Break cycles of emotional or reward-based eating.
• Sleep better and regulate stress.

Dr Abby Hyams completed her medical training in Bristol and has been a GP for 19 years, spending many of those as a partner in an NHS practice in Hemel Hempstead. She has a particular passion for supporting patients with weight loss and promoting positive behaviour change, helping individuals build sustainable habits that improve their long-term health and wellbeing.

• Wilding JPH et al. N Engl J Med 2021 (STEP 1)
• Jastreboff AM et al. N Engl J Med 2022 (SURMOUNT-1)
• Pi-Sunyer X et al. N Engl J Med 2015 (SCALE)
• Drucker DJ. Cell Metab 2018: Mechanisms of GLP-1 action.
• NICE NG248 (2023).
• EMA EPARs for Wegovy, Saxenda, Mounjaro.

This article is for general information only and should not be used for the diagnosis or treatment of medical conditions. Medic Spot Limited has used all reasonable care in compiling the information but makes no warranty as to its accuracy. Consult a doctor or other healthcare professional for diagnosis and treatment of medical conditions. In the event of an emergency, please call 999 for immediate assistance.

There is no guarantee of a specific weight loss medication being prescribed. Clinicians will review your online questionnaire alongside other independently validated medical information about you and will recommend the most appropriate weight loss treatments for you based on your answers. In some cases, the clinicians may contact you for additional information. See Terms of Service for more information.