Weight loss injections: how they work, the evidence, costs, eligibility and side effects

What the weight loss injection is and how it works

Behind the term weight loss injection are two active substances: semaglutide, a GLP-1 receptor agonist, and tirzepatide, which additionally acts on the GIP receptor. Both mimic gut hormones released after eating. They slow gastric emptying, strengthen the feeling of fullness in the brain and dampen appetite. People eat less on the therapy without constantly forcing themselves; that is exactly what distinguishes it from diets that fail because of hunger.

The substances originate from diabetes therapy. There it was noticed that patients lost considerable weight. Higher doses were then specifically studied and approved for obesity treatment: semaglutide 2.4 mg (in the EU since 2022) and tirzepatide (for weight management since 2023). Both are injected once a week under the skin with a pre-filled pen, usually into the abdomen or thigh.

Obesity is now understood as a chronic disease with a strong biological component; Blüher describes in Nature Reviews Endocrinology how genetics, hormones and environment regulate weight and why the body counteracts weight loss. This explains why medicines that intervene in this regulation are so much more effective than willpower alone, and why weight returns after stopping.

Detailed profiles of the individual medicines: Mounjaro (tirzepatide), Wegovy (semaglutide 2.4 mg) and Ozempic (semaglutide), which is approved for type 2 diabetes.

What the trials show

The figures from the approval trials are the basis of any honest consultation. All trials combined the substance with a diet and exercise programme; the placebo groups also received this programme.

Trial	Substance	Duration	Weight loss on drug	Placebo
STEP 1 (NEJM 2021)	Semaglutide 2.4 mg	68 weeks	14.9 %	2.4 %
STEP 5 (Nature Medicine 2022)	Semaglutide 2.4 mg	104 weeks	15.2 %	2.6 %
SURMOUNT-1 (NEJM 2022)	Tirzepatide 5 / 10 / 15 mg	72 weeks	15.0 / 19.5 / 20.9 %	3.1 %
SURMOUNT-3 (Nature Medicine 2023)	Tirzepatide after 12-week lifestyle programme	72 weeks	further 18.4 %	gain 2.5 %
SELECT (NEJM 2023)	Semaglutide 2.4 mg in heart disease	mean 40 months	cardiovascular events minus 20 %	comparison

Two results deserve particular attention. First, the effect persists as long as injections continue: STEP 5 showed a stable loss of around 15 percent after two years. Second, it is not only about kilograms: in the SELECT trial of 17,604 people with overweight and existing cardiovascular disease but without diabetes, semaglutide reduced the risk of heart attack, stroke or cardiovascular death by 20 percent.

The analysis by Xie, Choi and Al-Aly in Nature Medicine 2025 compared 175 health outcomes on GLP-1 therapy with other diabetes medicines in a large care cohort: fewer cardiovascular events, fewer substance use and psychotic disorders, fewer seizures, but more gastrointestinal complaints, drops in blood pressure, kidney stones and drug-induced pancreatitis. That is the realistic balance: great benefit, real risks that you need to know.

How semaglutide and tirzepatide compare head to head is covered in Wegovy or Mounjaro.

Who is eligible

Both substances are approved for weight management in adults with a BMI of at least 30, or at least 27 with at least one weight-related condition such as high blood pressure, type 2 diabetes, prediabetes, dyslipidaemia, sleep apnoea or cardiovascular disease. The S3 obesity guideline of the German Obesity Society sees medication as a component of an overall concept of dietary change, exercise and behavioural change when the basic programme alone is insufficient.

The therapy is not suitable during pregnancy and breastfeeding, with known hypersensitivity, with medullary thyroid carcinoma or multiple endocrine neoplasia type 2 in personal or family history, with acute pancreatitis and with severe gastrointestinal disease such as gastroparesis. Caution applies with gallstones, diabetic retinopathy and concurrent insulin or sulfonylurea therapy because of the risk of hypoglycaemia. Anyone with an eating disorder involving vomiting needs different treatment first.

What to expect in the consultation: height, weight, waist circumference, pre-existing conditions, medicines, pregnancy plans, blood values (fasting glucose or HbA1c, lipids, liver and kidney values, thyroid), a look at your previous attempts and a conversation about realistic goals.

Who may prescribe it and how eligibility, prescription and insurance work: Getting a weight loss injection prescribed.

Dose escalation and everyday life

Both substances are increased slowly so that the gastrointestinal tract adapts: semaglutide over 16 weeks from 0.25 mg to 2.4 mg, tirzepatide in steps of 2.5 mg every four weeks up to the individually suitable dose of 5, 10 or 15 mg. Anyone who tolerates a step poorly stays on it longer. Weight loss is fastest in the first months and flattens after about a year; that is normal and not a sign that the effect is waning.

Practical rules that noticeably reduce side effects: small portions, eat slowly, avoid fatty and heavily spiced food at first, drink enough, reduce alcohol, do not lie down when nauseous. Protein-rich food and strength training preserve muscle; without training part of the weight loss comes from muscle mass. Missed injections can be made up within five days; after that the dose is skipped.

Putting side effects and risks in perspective

In STEP 1, 44 percent of participants reported nausea, 32 percent diarrhoea, 24 percent vomiting and 23 percent constipation; on placebo the figures were 17, 16, 7 and 10 percent. Most complaints were mild to moderate and occurred during dose escalation. Seven percent discontinued because of side effects, three percent on placebo. The figures for tirzepatide are similar.

Less common but relevant: gallstones and gallbladder inflammation (more frequent with rapid weight loss), pancreatitis, reflux, hair loss as a consequence of rapid weight loss, drops in blood pressure and dizziness, increased effect of other medicines through delayed gastric emptying. On the question of depression and suicidal thoughts, the European Medicines Agency’s safety committee found no causal link after review in 2024; anyone with a history of mental illness should nevertheless mention it in the consultation so that the course is monitored.

Before operations under anaesthesia, inform the anaesthesia team about the therapy because the stomach empties more slowly.

All side effects with frequencies and warning signs: Side effects of weight loss injections.

Stopping: what happens afterwards

The weight loss injection does not cure obesity, it treats it, similar to a blood pressure drug treating blood pressure. In the STEP 4 trial all participants first lost weight for 20 weeks with semaglutide; then some switched to placebo. This group regained 6.9 percent over the following 48 weeks, while the semaglutide group lost a further 7.9 percent. Follow-ups show that one year after stopping, on average about two thirds of the lost weight has returned; blood pressure and lipids also approach baseline again.

For your decision this means: plan the therapy long-term, use the time on the drug to change diet, exercise and sleep permanently, and discuss with your doctor what a later taper or maintenance dose might look like. Anyone who sees the injection as a short course will be disappointed.

Trial data, tapering and what helps keep the weight off: Stopping weight loss injections.

Costs: what you really pay

German statutory health insurers may not reimburse medicines that primarily serve weight reduction; section 34(1) SGB V counts them among excluded lifestyle drugs. An exception applies only to the diabetes indication. Private health insurers decide according to tariff and medical necessity; a cost commitment in advance is worthwhile.

The products cost roughly 200 to 400 euros per month at the pharmacy depending on substance and dose level; pharmacy prices change, the information from your pharmacy is binding. Add the medical services. OpenClinic24 bills under the German fee schedule for doctors: at booking, €10.72 is reserved (GOÄ no. 1), the items for the initial consultation and follow-up checks appear on the invoice with code, factor and amount, and you see the price of your programme before confirming on the weight loss page.

Beware of offers without medical supervision and of sources outside pharmacies: counterfeit pens are a real problem, and without monitoring, side effects and contraindications go unrecognised.

What else helps, with and without the injection

The medication works best as part of a programme. Three components have consistently shown effect in studies: an energy-reduced, protein-focused diet with plenty of vegetables and few sugary drinks, at least 150 minutes of moderate exercise per week plus two strength sessions, and enough sleep, because sleep deprivation increases hunger and insulin resistance. Anyone also suffering from sleep apnoea, depression or thyroid problems should have these conditions treated; otherwise they slow every attempt.

A word on expectations: five percent weight loss already measurably improves blood pressure, blood sugar and joint complaints. Ten to fifteen percent fundamentally changes risks. The goal is health, not a number on the scales.