Are the New Weight Loss Injections Safe — and What Should You Know Before Starting?

You've seen the headlines. Here's what the clinical record actually shows.

Picture this: you've spent ten minutes on a search engine and come away with two completely contradictory impressions. One set of results says these injections are transformative; another warns of hospitalisations and black-market fakes. Both things are, in their own way, true, which is precisely why the detail matters.

Tirzepatide and semaglutide belong to a class of medicines called GLP-1 receptor agonists (tirzepatide also activates GIP receptors, making it a dual agonist). They work by mimicking gut hormones that regulate appetite and slow gastric emptying. The mechanisms behind the new weight loss injections are well-studied at this point: tirzepatide's phase-three SURMOUNT programme involved thousands of adults across multiple trials, and semaglutide's STEP programme was similarly large-scale. The NHS has published patient-level safety information for both tirzepatide and semaglutide, drawing on that clinical evidence base.

Both medicines carry the MHRA's Black Triangle (▼) designation, meaning they are subject to additional monitoring as post-market safety data accumulates. That is standard for newer medicines. It is not a signal of unusual danger; it is the regulatory system doing its job.

The side effects most people actually experience

The side-effect profile of these medicines is dominated by the gut. Nausea is the most frequently reported, followed by diarrhoea, constipation, reflux, and burping. Some people add fatigue or mild headaches in the first couple of weeks. Injection-site reactions (redness, mild soreness) are possible with any subcutaneous injection.

The timing matters. These effects are most pronounced when starting treatment or stepping up to a higher dose, and they tend to settle as the body adjusts, usually within a week or two. The dose-escalation schedule that prescribers follow (both medicines start low and increase gradually) exists specifically to reduce this burden. That gradual approach is a safety feature, not a formality.

Most people do not stop treatment because of side effects. In the SURMOUNT-1 and STEP-1 trials, discontinuation rates due to adverse events were modest, even at the highest doses. That said, individual experience varies. Some people sail through the first pen; others find the first few weeks genuinely uncomfortable. A prescriber who can discuss your history before you start (and who is reachable if things feel wrong) makes a measurable difference to how safely you navigate that period. Our clinical team at nume (sorry, at nume, reviewed by our GPhC-registered prescribers) provides 7-day aftercare for exactly this reason.

When to get medical help without delay

Most side effects are uncomfortable rather than dangerous. A handful are not, and you should know them before your first dose.

Pancreatitis is a serious but infrequent risk. In January 2026 the MHRA issued a Drug Safety Update specifically highlighting acute pancreatitis as a known effect of GLP-1 medicines that warrants prompt attention. The sign to act on: severe, persistent stomach pain (pain that does not ease and may radiate through to your back) with or without vomiting. Do not wait to see if it settles. Get emergency medical help.

Severe dehydration can follow persistent vomiting or diarrhoea. If you cannot keep fluids down for more than 24 hours, seek medical advice; prolonged dehydration can affect kidney function.

Gallbladder symptoms (sharp upper-right abdominal pain, pain after eating fatty food, jaundice) should prompt a call to your GP or 111.

Allergic reactions: rash, swelling of the face or throat, difficulty breathing, seek emergency care immediately.

Any side effect that feels wrong or outside what your prescriber described should go back to your clinical team. Suspected reactions can also be reported to the MHRA directly at the Yellow Card scheme, which exists to build the long-term safety picture for all patients using these medicines.

For a fuller breakdown of individual side-effect patterns, the side effects associated with these medicines are covered in more depth in a dedicated guide.

What a safe clinical process looks like before you start

The safety of these medicines is not separable from the safety of how they are prescribed. If you have questions about whether weight loss injections are safe as a category, it helps to understand that counterfeit pens (seized in significant numbers by the MHRA, some containing insulin rather than the labelled medicine) exist precisely because demand has outrun proper clinical channels. Buying from an unverified seller, or obtaining a supply without a clinical assessment, removes every safeguard these medicines were designed to sit within.

A legitimate process includes: a prescriber reviewing your medical history, current medicines, and BMI; identity and weight verification; and clinical sign-off before any medicine is issued. At nume, every consultation is reviewed the same day by a GPhC-registered Independent Prescriber, a person, not automated software. You can verify our pharmacy on the GPhC register (registration 9012878). That step takes about 30 seconds and is the single most useful check any patient can make about any online pharmacy.

If you're considering whether treatment might be appropriate for your situation, our prescribers cover suitability, contraindications and expected side effects as part of the consultation itself, and our dedicated guides on whether the new weight loss injection is safe and whether the weight loss injection is safe for you specifically are a good place to read ahead before that conversation. Check your eligibility and start that conversation, it costs nothing and commits you to nothing.