Weight Loss Injection Warnings You Shouldn't Ignore

The biggest misconception: warnings mean the medicine is dangerous for everyone

People sometimes read the side-effect leaflet for tirzepatide or semaglutide and close the box. The list of possible effects can look alarming at first glance, and that reaction is completely understandable. But a warning listed in a patient leaflet is not a prediction. It is a disclosure, drawn from clinical trials involving thousands of participants, of every effect that appeared at a higher-than-background rate. Most people who use these medicines do not experience most of the listed effects.

The NHS patient information for tirzepatide sets this out clearly: the most common effects are gastrointestinal, including nausea, constipation, diarrhoea and indigestion. These tend to be most noticeable in the early weeks and usually ease as the body adjusts. The graduated dosing schedule, starting at a low tolerability dose before any therapeutic step-up, exists specifically to reduce this burden. If you open the leaflet expecting a list of certainties, you will misread it. If you read it as a probability guide, it becomes genuinely useful.

Both Mounjaro (tirzepatide) and Wegovy (semaglutide) carry a Black Triangle designation from the MHRA, meaning they are subject to additional monitoring. That is a feature of newer medicines, not a sign that something is wrong. It means regulators continue collecting data after approval, which is exactly what you want from a system that takes post-market safety seriously. You can learn more about how these medicines are regulated on our weight loss treatment overview.

The MHRA warnings that do need your full attention

Some warnings carry more weight than others, and two in particular deserve to be understood precisely rather than passed over.

First: pancreatitis. In January 2026 the MHRA issued a Drug Safety Update confirming that acute pancreatitis is a known, infrequent but potentially serious risk with GLP-1 receptor agonist medicines. The symptom to act on is severe, persistent stomach pain that may spread through to your back, with or without vomiting. This is not the normal nausea of starting a new dose. If you experience it, stop the injection and get medical help that day. Do not wait to see if it settles. Our FAQs page covers what to do if you are unsure about a symptom while on treatment.

Second: contraception. Women using oral contraceptives alongside tirzepatide should add a non-oral method such as condoms for the first four weeks of treatment and for four weeks after each dose increase, because GLP-1 medicines slow gastric emptying and may reduce pill absorption. NHS guidance does not flag the same interaction for semaglutide, but this is an area worth discussing with your prescriber rather than assuming. The same mechanism that makes these medicines effective at slowing digestion can affect other medicines taken at the same time.

A third signal worth naming, noted in NHS guidance: tell your anaesthetist and surgical team before any procedure that you are taking a GLP-1 medicine. Gastric emptying is relevant to anaesthetic safety. This is not a reason to stop treatment but it is information your surgical team needs in advance. The clinical concerns raised by UK doctors on this point are worth reading if you have planned surgery.

The warning that has nothing to do with the medicine itself: counterfeit pens

The MHRA's enforcement activities through 2024 and 2025 turned up approximately 20 million doses of illegally traded weight loss medicines with a street value around £45 million, including the first known UK site manufacturing counterfeit tirzepatide pens, raided in Northampton in October 2025. Fake pens seized in earlier operations were found to contain insulin, causing serious harm to people who had no idea they were injecting the wrong medicine.

This is the warning that sits entirely outside the clinical leaflet. It is not about the medicine; it is about what you actually receive when you buy from an unverified source. The red flags are consistent: sellers offering these medicines without a prescription through social media, prices far below the market rate, or discount codes from unknown accounts. You can verify any online pharmacy on the GPhC pharmacy register before ordering. Suspect sellers can be reported via the MHRA's Yellow Card scheme. Genuine supply through a GPhC-registered pharmacy is not just a compliance point, it is the difference between injecting a licensed medicine and injecting something unknown.

For a broader look at what these medicines actually are and how they are obtained through a legitimate route, our guide to weight loss injections in the UK covers the process from consultation to delivery. If you have questions about what the training and clinical process looks like for prescribers, the injection training guidance explains what regulated providers follow.

When you should genuinely pause before using a weight loss injection

There are clinical situations where these medicines are not appropriate, and these are not buried small print. Mounjaro and Wegovy are not licensed for use during pregnancy, while breastfeeding, or in people who are trying to conceive. They are not licensed for under-18s. People with a personal or family history of medullary thyroid carcinoma or a condition called MEN2 syndrome should not use them. A history of pancreatitis or certain gastrointestinal conditions also requires careful prescriber assessment before treatment starts.

None of these are reasons to dismiss GLP-1 medicines as a category. They are reasons why a clinical assessment exists. A prescriber reviewing your case is not a formality between you and the medicine, it is the process that determines whether the medicine is right for you, and at what dose and pace. The cost of that process at a reputable provider includes the prescription, delivery and ongoing aftercare as standard; it is not charged separately as an add-on.

At nume, every consultation is reviewed by a GPhC-registered Independent Prescriber, a real clinician reading your answers, not an automated filter. If treatment is clinically suitable, it is dispatched the same day for free next-working-day delivery. You can find out more about who we are and how the service is run, or meet the clinical team behind it. If you have come across claims linking tanning injections to weight loss, our prescribers can explain what is actually licensed and why those products sit outside regulated treatment. If you are weighing up your options, our comparison of three weight loss injections currently available sets out how tirzepatide, semaglutide and liraglutide differ in practice. If you are ready to find out whether treatment is right for you, speak to our prescribers through a free consultation.