Is it safe to take injections for weight loss?

Step 1: understanding what 'licensed and monitored' actually means

A medicine being licensed does not mean risk-free. It means regulators have reviewed the evidence and concluded the benefits outweigh the risks for the population the licence covers. For Mounjaro and Wegovy, that population is adults with a BMI of 30 or above, or 27 or above alongside at least one weight-related health condition such as high blood pressure, high cholesterol or type 2 diabetes. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance.

Both medicines carry what the MHRA calls a Black Triangle designation, written as ▼. This means they are subject to additional monitoring, and any suspected side effects reported by patients or clinicians feed directly into ongoing safety surveillance. You can read a plain summary of what monitoring looks like in practice on the NHS medicines page for tirzepatide. The system exists precisely because the evidence base continues to grow after a medicine reaches patients.

What this means for you practically: taking either injection under a prescriber's ongoing supervision, with regular clinical reviews before each repeat, is a very different situation from obtaining them from an unverified source. The supervision is part of the safety, not optional extra.

Step 2: knowing the side effects before you start

Most people who take weight-loss injections experience some gastrointestinal effects, particularly in the early weeks. Nausea is the most frequently reported. Others include vomiting, loose stools, constipation, indigestion and burping. These tend to cluster around dose increases and usually ease as your body adjusts. Fatigue, mild headaches and dizziness are also reported, less often.

Injection-site reactions (redness, itchiness or a small lump at the site) can occur with any subcutaneous injection. Rotating between the abdomen, thigh and upper arm, and cleaning the area first, reduces the likelihood. If you'd like to understand the basics of safe injection technique, our weight-loss injection guide covers the practical steps.

In January 2026, the MHRA updated its safety guidance on GLP-1 receptor agonists to flag acute pancreatitis as a known side effect. It is infrequent, but it can be serious. Gallbladder problems are also listed in the prescribing information for both medicines. If you want a fuller picture before deciding, our page on whether it is safe to use weight loss injections sets out what the evidence shows. Knowing these risks before you start (and knowing what symptoms to look for) is the responsible approach, not a reason to avoid treatment if you're a suitable candidate.

Step 3: recognising symptoms that need prompt medical attention

Most side effects from weight-loss injections are manageable and temporary. A small number of symptoms warrant urgent attention, and it matters to know them before your first pen arrives.

Seek medical help promptly if you experience: severe or persistent stomach pain, especially pain that spreads to your back and may be accompanied by vomiting (this can indicate pancreatitis); signs of a serious allergic reaction such as swelling of the face, lips or throat, difficulty breathing, or a rapid heartbeat; symptoms of gallbladder problems including pain in the upper right abdomen, fever or yellowing of the skin; or significant dehydration caused by repeated vomiting or diarrhoea.

If you experience low mood or thoughts of self-harm, contact your prescriber or GP promptly. These are listed in the patient information leaflets for both medicines.

You can also report any suspected side effect directly to the MHRA using its Yellow Card scheme. This is open to patients as well as healthcare professionals, and the reports genuinely shape ongoing safety monitoring. A question our prescribers hear most weeks is whether the weight loss injection is safe, and the honest answer is that it depends on the individual, which is why that monitoring matters so much.

Step 4: who shouldn't use these injections, and what the consultation resolves

Weight-loss injections are not suitable for everyone. Neither Mounjaro nor Wegovy is recommended during pregnancy, while breastfeeding, or for anyone trying to conceive. They are not licensed for under-18s. A personal or close family history of medullary thyroid cancer or multiple endocrine neoplasia type 2 rules out tirzepatide. Certain gastrointestinal conditions and a personal history of pancreatitis also require careful prescriber consideration.

If you take the oral contraceptive pill, there is specific guidance worth knowing: during the first four weeks of Mounjaro treatment and for four weeks after each dose increase, reduced pill absorption is possible, and adding a barrier method is advised. The NHS England guidance on weight management injections covers this in detail, including advice on HRT and surgery.

None of these exclusions can be worked out from a BMI calculator alone. That is exactly what a clinical consultation is for. At nume, a GPhC-registered Independent Prescriber reads every consultation personally before anything is approved. If you're planning to order around a holiday or a busy patch at work, ordering before 12pm on a working day means dispatch happens the same day once you're clinically approved, and your pen arrives the next working day. You can explore all of our weight-loss treatments and what the process involves before you start. There's also a broader look at the safety evidence for weight-loss injections if you want more context before deciding, and if you're specifically weighing up a newer option, our guidance on whether the new weight loss injection is safe is worth reading alongside it. People also often ask us whether taking weight loss injections is safe as a general starting point, and our answer there covers the key factors a prescriber will consider. Our FAQs cover the questions people ask most. And if anything on this page raises a specific concern, our team is reachable through our contact page seven days a week.