GLP-1 Weight Loss Injections: How Semaglutide Works in the Body and Who Should Avoid Them
What GLP-1 Actually Is
Glucagon-like peptide-1 is a hormone your small intestine already makes. It releases after you eat, signals the pancreas to produce insulin, slows how fast food leaves your stomach, and tells the brain's hypothalamus that you've had enough. GLP-1 receptor agonists, drugs like semaglutide (sold as Ozempic for diabetes and Wegovy for weight management) and liraglutide (Saxenda), mimic this hormone at a far higher concentration and for far longer than your body would sustain on its own.
The natural GLP-1 your gut produces has a half-life of about two minutes. Semaglutide's half-life is approximately seven days. That single difference is what makes weekly injections possible and what drives the sustained appetite suppression that short-acting versions cannot achieve.
What Happens After You Inject
Within hours of an injection, gastric emptying slows. Food sits in the stomach longer, which extends the feeling of fullness and blunts post-meal blood sugar spikes. The pancreas reads the GLP-1 signal and increases insulin secretion only when glucose is actually present, a glucose-dependent mechanism that sharply reduces the risk of hypoglycaemia compared to older diabetes drugs.
In the brain, GLP-1 receptors in the hypothalamus and brainstem register the drug's presence and reduce hunger signalling. Some research suggests the drug also acts on reward pathways, which may explain why patients report reduced cravings for alcohol and ultra-processed foods, effects that go beyond simple calorie counting. The dosage is typically titrated upward over several weeks precisely because the gastrointestinal side effects (nausea, vomiting, constipation) are dose-dependent and the body needs time to adjust.
The Weight Loss Numbers
The STEP 1 trial, published in the New England Journal of Medicine in 2021, followed 1,961 adults with obesity over 68 weeks. Participants on 2.4 mg weekly semaglutide lost an average of 14.9% of their body weight versus 2.4% on placebo. That is the largest sustained weight loss ever recorded in a drug trial without surgical intervention.
Indian data is catching up. A 2023 real-world analysis from AIIMS Delhi, presented at the annual Endocrine Society of India conference, found that Indian patients with obesity and Type 2 diabetes on semaglutide showed comparable glycaemic improvement to global trials, though absolute weight loss was modestly lower, consistent with known differences in body composition and BMI thresholds at which metabolic risk rises in South Asian populations. Indian clinical guidelines now place the BMI cut-off for obesity intervention lower than the global standard (23 kg/m² for risk assessment versus 25 kg/m²), which means a larger share of the Indian population technically qualifies for these drugs than the global numbers suggest.
Who Should Not Take GLP-1 Injections
Contraindications are specific and non-negotiable, and several are frequently missed in the rush of enthusiasm around these drugs.
A personal or family history of medullary thyroid carcinoma is the clearest hard stop. GLP-1 receptor agonists caused thyroid C-cell tumours in rodent studies, and while a direct causal link in humans has not been established, regulatory agencies including the US FDA and India's CDSCO carry a black-box warning. Anyone with Multiple Endocrine Neoplasia syndrome type 2 (MEN2) is also excluded.
A history of pancreatitis, acute or chronic, is another firm contraindication. GLP-1 drugs are associated with increased pancreatic enzyme levels, and in someone with prior pancreatic damage, the risk of triggering another episode is real. Gallbladder disease has also emerged as a concern: rapid weight loss from any cause increases gallstone formation, and semaglutide appears to compound this.
Pregnancy and breastfeeding rule these drugs out entirely. Animal studies show foetal harm at therapeutic doses, and there is no human safety data.
People with a history of eating disorders, anorexia nervosa in particular, require extreme caution. The appetite suppression these drugs produce can reinforce restrictive behaviours in ways that are difficult to monitor and clinically dangerous.
Type 1 diabetes is not an approved indication. GLP-1 drugs work through a glucose-dependent insulin pathway; in Type 1, where the pancreas produces no insulin at all, this mechanism offers no meaningful benefit and the risk profile is unfavourable.
Severe kidney or liver impairment requires dose adjustment at minimum and may preclude use altogether, since both organs are involved in drug clearance. Patients on insulin or sulfonylureas need careful monitoring because the combination can cause blood sugar to drop too low, even though semaglutide alone rarely causes hypoglycaemia.
What the Drug Cannot Do
GLP-1 injections do not fix the conditions that drive obesity, disrupted sleep, ultra-processed food environments, sedentary work, chronic stress, and the insulin resistance that builds over years. The STEP 1 trial also showed that participants who stopped semaglutide regained two-thirds of their lost weight within a year. The drug works while you take it. That is not a flaw in the drug; it is a fact about the biology of weight regulation that no drug has yet changed.
Patients who expect injections to substitute for dietary change consistently show worse outcomes than those who combine the drug with structured eating and movement. The mechanism is powerful. The context in which it is used determines whether that power translates into durable health or a temporary number on a scale.
The real question these drugs force is not whether they work, the trial data is unambiguous, but whether the person taking them has been screened carefully enough to be safe, and whether the clinical support around them is substantial enough to make the loss last.