How Weight Loss Medications Work

Modern weight-loss medicines work with your body’s natural hormones that control hunger and digestion. They help you feel full sooner, eat less, and keep blood-sugar levels steady. The best results come when they’re used alongside balanced eating, regular activity, good sleep, and behaviour change.

Weight-loss medications act mainly on hormones called incretins, especially GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide). These hormones signal between your gut, brain, and pancreas to regulate appetite and metabolism.
Medicines that mimic them, such as semaglutide, tirzepatide, and liraglutide, lower hunger signals, slow stomach emptying, and improve how the body handles glucose.

Your brain and digestive system constantly talk to each other about how hungry or full you are. Signals from your stomach and fat stores tell the brain when to start and stop eating.

• The hypothalamus acts as the control centre.
• Leptin and insulin show long-term energy stores; ghrelin rises before meals to trigger hunger.
• After eating, GLP-1, PYY, and CCK are released from the gut, promoting fullness.
• Stress, poor sleep, and highly processed foods can override these signals, keeping appetite high.

Incretins are hormones released after eating. They help the pancreas release insulin only when glucose is high enough, preventing dangerous drops in blood sugar.

GLP-1 and GIP amplify insulin secretion and reduce glucagon. When these hormones are recreated as medicines, they help stabilise glucose, reduce appetite, and support weight loss.

They slow how fast your stomach empties, make you feel satisfied sooner, reduce cravings, and help your body use insulin more effectively.

• Stomach: slower emptying keeps food in longer, stretching the stomach and triggering fullness.
• Brain: GLP-1 receptors in appetite centres reduce food-cue response.
• Pancreas: boosts insulin when glucose is high and lowers glucagon, reducing post-meal spikes.

Most people notice reduced hunger within a few weeks and steady weight loss over months. Average reduction is about 10–15 % with GLP-1s and up to 22 % with dual agonists.

• Weight change is gradual and plateaus after 6–12 months.
• Maintenance depends on ongoing habits.
• Other benefits include lower blood sugar, blood pressure, and improved cholesterol.

Most side effects happen at the start and improve with slow dose increases. Nausea, constipation, or mild stomach upset are common.

• Start low and go slow.
• Eat smaller meals, drink water often, prioritise protein and cooked vegetables.
• Call your clinician if pain is severe, you vomit often, or can’t keep fluids down.

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.

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.