GLP-1 Weight-Loss Pens and Tirzepatide: Who They Suit and the Risks

Over the past few years, almost every week a patient asks me about "weight-loss pens". Some heard about them from friends; some bought them and injected themselves, then vomited so badly they had to come and see me. I want to give you straightforward information: these medicines genuinely help a lot, but they are not a shortcut without a price, and they don't suit everyone.

What are weight-loss pens and how do they work?

Weight-loss pens are prefilled pen injections given under the skin, mostly once a week. There are currently two main groups of medicine:

• GLP-1 receptor agonists, such as semaglutide (Wegovy for weight loss, Ozempic for diabetes) and liraglutide, an older medicine injected every day
• GIP/GLP-1 receptor agonist: tirzepatide (Mounjaro), which became available in Thailand from mid-2025

These medicines mimic hormones from the gut, so you feel full sooner and for longer, your appetite falls, your stomach empties more slowly, and blood sugar is better controlled.

How well do they work? Evidence from the key trials

There is also evidence that they help obstructive sleep apnea in people with obesity and slow kidney disease in people with diabetes. These figures are "averages": some people lose a lot, others lose little.

Who they suit, and who they don't

In general, doctors consider them for adults who have:

• A body mass index (BMI) of 30 or more, or
• A BMI of 27 or more together with a weight-related condition, such as diabetes, high blood pressure, high cholesterol, fatty liver, knee osteoarthritis or sleep apnea
• In Asian people, doctors may use lower thresholds, because the risk of metabolic disease appears at a lower BMI than in Western populations

They should not be used, or need great caution, in these situations:

• A personal or family history of medullary thyroid cancer (MTC) or MEN2 syndrome
• Pregnancy, planning pregnancy or breastfeeding (stop the medicine before conceiving, as your doctor advises)
• Previous pancreatitis, gallbladder disease or severe gastroparesis (slow stomach emptying)
• A past or current eating disorder, such as anorexia or bulimia
• People who want to lose a small amount of weight for appearance without a medical indication

Side effects and risks you need to know

Common: nausea, vomiting, bloating, constipation and diarrhea, usually when starting or increasing the dose. That's why doctors increase the dose gradually every 4 weeks.

Less common but important:

• Gallstones and gallbladder inflammation, especially with rapid weight loss
• Pancreatitis: severe abdominal pain spreading to the back, with vomiting
• Dehydration from vomiting or diarrhea, which can worsen kidney function
• Low blood sugar, if used together with insulin or sulfonylurea medicines
• NAION, a loss of blood supply to the optic nerve, which the European Medicines Agency (EMA) concluded in 2025 is a very rare side effect of semaglutide. Sudden blurred vision or loss of vision needs a doctor immediately.
• A risk of aspiration under anesthesia, because the stomach empties slowly. Always tell your doctor before any surgery or endoscopy.
• Tirzepatide may reduce the effectiveness of oral contraceptive pills when starting and increasing the dose, so use an additional method of contraception.

Muscle loss: part of the weight lost is muscle, so you need enough protein and strength training alongside. See Nutrition after 50 and How much can TRT build muscle, and how to build muscle properly.

What happens when you stop?

The SURMOUNT-4 study and the extension of STEP-1 found that when the medicine is stopped, most of the weight usually comes back within a year, because the medicine doesn't permanently change hunger signals. Obesity is therefore seen as a chronic disease: some people need long-term treatment, while others can reduce the dose once their habits have changed. Planning with your doctor from the start is important.

Weight, hormones and sexual health

Losing weight and belly fat raises testosterone in many men with obesity and helps erection problems related to blood vessels. Read more in Testosterone with diabetes, high blood pressure and chronic disease and Healthy fats: how to choose.

Care at Dr. Kris Clinic

Before starting, I assess your history, chronic conditions and current medicines, arrange necessary blood tests such as blood sugar, cholesterol, liver and kidney function, and discuss realistic goals. During treatment I follow up on side effects, adjust the dose and plan your diet and exercise. Everything is done only as assessed and prescribed by a doctor. English-speaking consultations are available.

Go to hospital immediately if you have severe abdominal pain that doesn't go away, vomiting so you can't eat or drink, yellow skin or eyes, sudden blurred vision or loss of vision, or faintness and palpitations from low blood sugar.