The real cons of semaglutide, according to clinical evidence

The gastrointestinal burden: what the STEP 1 data showed

The STEP 1 trial followed 1,961 adults with obesity over 68 weeks. Average weight loss at the 2.4mg maintenance dose was around 15% of body weight, a clinically meaningful result. But the same dataset tells the other side of the story: gastrointestinal adverse events were reported by the large majority of participants taking semaglutide, with nausea being the single most common complaint. Vomiting, diarrhoea, and constipation also appeared at rates substantially higher than in the placebo group.

These effects tend to be most pronounced after starting treatment and again after each dose step upward. For most people they ease within one to two weeks as the body adjusts. For some, though, they persist or are severe enough to prompt a pause or a slower titration. That is not a failure, it is a well-understood pharmacological pattern, and it is exactly the kind of thing a prescriber monitors. The NHS semaglutide page lists these effects clearly and explains when to seek help.

Practical note: nausea often hits harder on an empty stomach or if meals are too large. Smaller, lower-fat portions tend to help. Personal adjustments like that are worth discussing with the clinical team rather than quietly pushing through.

Serious risks that belong in the conversation

Most people tolerate semaglutide well once they are established on their dose. That said, the regulatory record includes risks that deserve honest attention. In January 2026, the MHRA published a Drug Safety Update specifically addressing acute pancreatitis as a known, infrequent but potentially serious effect of GLP-1 medicines including semaglutide. Severe, persistent stomach pain that spreads toward the back (with or without vomiting) needs same-day medical attention, not a wait-and-see approach.

Gallbladder problems are a second flagged risk. Rapid weight loss of any kind can increase the likelihood of gallstones, and semaglutide is no exception. Symptoms of a gallbladder episode (sharp pain in the upper right abdomen, nausea, fever) should be assessed promptly by a doctor. Suspected side effects from any medicine can be reported directly to the MHRA via the Yellow Card scheme.

There are also populations for whom semaglutide is not recommended at all: pregnancy, breastfeeding, and the period when someone is trying to conceive. People under 18 are outside the licensed indication. A personal or family history of medullary thyroid carcinoma or MEN2 requires particular prescriber scrutiny. These are not bureaucratic hurdles, they reflect genuine biological reasons for caution.

The dependency question: what happens when treatment stops

This is the con that catches people off guard. Semaglutide suppresses appetite partly by slowing how quickly the stomach empties and by acting on hunger signals in the brain. When the medicine stops, those mechanisms reverse. Clinical follow-up data consistently show that a significant proportion of weight lost during treatment is regained within one to two years of stopping, absent sustained lifestyle change.

That does not make semaglutide ineffective, it makes it more like a blood pressure medicine than a course of antibiotics. The NICE appraisal of semaglutide (TA875) reflects this: NHS use is recommended for a maximum of two years, and only within a specialist weight management service that includes structured support around diet and activity. The expectation is that the window of reduced appetite is used to build habits, not that the medicine does the work alone.

For people exploring the specific drawbacks of Wegovy versus other options, or thinking through whether a GLP-1 medicine suits their situation, the question of long-term use and what comes after treatment is worth exploring fully at consultation. There is also a cost dimension to sustained treatment that is worth understanding, you can read about what Wegovy typically costs in the UK to get a realistic picture before you start.

Practical constraints: access, monitoring, and what the prescription process involves

Semaglutide is a prescription-only medicine. That is not a technicality, it reflects the fact that it interacts with other conditions and medicines, requires dose titration by a qualified clinician, and needs monitoring over time. Anyone offering it without a clinical assessment is operating outside the law, and the MHRA has seized large quantities of fake GLP-1 pens sold through unverified channels.

For women using oral contraceptives, the interaction profile for semaglutide differs from tirzepatide, the NHS notes no equivalent evidence of reduced pill absorption for semaglutide, though prescribers will review your full medicines list regardless. If you want a broader picture of how semaglutide works as a medicine before weighing its downsides, the semaglutide overview page covers the mechanism and licensed uses. For those already into their treatment and looking at specific doses, the pages on semaglutide 1mg, semaglutide 2mg, and semaglutide 3mg go into detail about those points in the titration schedule.

The monitoring piece matters practically. If you order in December and a bank holiday falls mid-month, delivery timing shifts, planning your repeat prescription around those dates is worth a quick conversation with the clinical team beforehand. At nume, every repeat order gets a fresh clinical review by a GPhC-registered prescriber before dispatch, which is how things like new symptoms or dose-change questions get caught early rather than missed. If you are weighing up the full picture and want to talk through whether semaglutide is right for you, speaking to our prescribers starts with a free consultation.