Weight-loss pens: what they are, how well they work, and what to watch for
Weight-loss pens are GLP-1 injectable medicines such as semaglutide and tirzepatide — prescription drugs, not supplements. In large trials, semaglutide 2.4 mg produced an average 14.9% weight loss over 68 weeks and tirzepatide 15 mg 20.9% over 72 weeks. Gastrointestinal side effects are common, and most of the weight returns within a year of stopping.
This article is information, not medical advice, and it is neither an argument for using these drugs nor against. They are prescribed and monitored by a doctor, and that is the only route.
What are they, and how do they differ from a supplement?
They are medicines. The drugs in this class imitate GLP-1, a hormone the gut releases after eating, producing earlier fullness, longer fullness, and less appetite. They are sold as pens because they are injected under the skin, usually once a week.
The difference from a supplement is not just strength — it is legal and it is about accountability. A drug has to pass trials in thousands of people and be approved before sale, side effects are tracked continuously, and a doctor decides who it suits.
How well do they work? The trial numbers
- Semaglutide 2.4 mg — STEP 1 (Wilding 2021), 1,961 people over 68 weeks, average weight loss 14.9% against 2.4% on placebo.
- Tirzepatide 15 mg — SURMOUNT-1 (Jastreboff 2022), 2,539 people over 72 weeks, average loss 20.9%.
- Cardiovascular effect — SELECT (2023) found semaglutide reduced major cardiovascular events by around 20% in people with existing heart disease and overweight or obesity, without diabetes.
These figures are clearly higher than anything other approaches achieve, which is why this drug class genuinely changed the picture.
What are the common side effects?
Nausea, vomiting, diarrhoea, constipation and abdominal pain are the most frequent, and the most common reason people stop. Most occur while the dose is being increased and ease over time.
Less common but worth knowing: gallstones, associated with rapid weight loss; pancreatitis, which is uncommon but serious; and thyroid C-cell tumours seen in rodents, which is why the drugs are contraindicated in anyone with a personal or family history of medullary thyroid cancer or MEN2 syndrome.
Do you lose muscle too?
Yes, and it is discussed less than it should be. Across trials of this class, the share of weight lost that is fat-free mass falls somewhere between about a quarter and forty per cent — comparable to losing weight by other means at the same rate.
The remedy is the ordinary one: enough protein and resistance training while on the drug. The medication reduces appetite; it does not choose what leaves.
Does the weight come back after stopping?
For most people, yes. The extension of STEP 1 found roughly two-thirds of the lost weight regained one year after stopping.
That is not a failure of the drug so much as the nature of obesity as a chronic condition — much like blood pressure rising again when the tablet stops. It means starting is a long-term decision rather than a three-month course, and that the time on the drug is the time to build eating and training habits that will outlast it.
How big is the counterfeit problem?
Large. The World Health Organization issued an alert about falsified semaglutide found in several regions from 2023, and there are documented cases of harm from pens bought online — in some instances insulin in counterfeit packaging, which causes severe hypoglycaemia.
There is one safe rule: obtain it from a doctor and a traceable pharmacy. Anything sold through social media or a chat group could contain anything at all.
Reading something you can act on beats reading something you cannot.