When weight loss injections aren't suitable: contraindications explained

You've been reading about these treatments for weeks — here's what the SmPC actually says

Most people who research Mounjaro or Wegovy online reach the prescribing information late, if at all. The Summary of Product Characteristics for each medicine, published on the electronic Medicines Compendium (eMC), lists the formal contraindications: the conditions under which the medicine must not be used, not just conditions that need monitoring.

For tirzepatide (Mounjaro), the hard contraindications are a personal or family history of medullary thyroid carcinoma, a diagnosis of multiple endocrine neoplasia type 2, and known hypersensitivity to tirzepatide or any ingredient in the pen. These are not grey areas. If any of them apply, the medicine is not prescribed. For semaglutide (Wegovy), the contraindications follow the same pattern: MTC history, MEN2, and hypersensitivity. Neither medicine has been shown to cause thyroid tumours in humans, but the signal emerged in rodent studies and the caution is written into the licence as a result.

Beyond those absolute bars, both SmPCs list situations that are not formal contraindications but are treated as clinical reasons not to proceed unless the picture is carefully assessed: severe gastrointestinal disease, a history of pancreatitis, and diabetic gastroparesis. A prescriber reading your consultation isn't just ticking a box, they're weighing a clinical picture. You can explore what the licensed treatments for weight management and diabetes look like side by side, because the two licences carry some differences worth understanding.

The situations that come up most in practice: pregnancy, pancreatitis, and mental health

Pregnancy is consistently the clearest practical contraindication. Neither Mounjaro nor Wegovy is recommended during pregnancy, while breastfeeding, or if you are actively trying to conceive. The NHS medicines page for tirzepatide is explicit on this, and the same applies to semaglutide. Animal studies showed potential harm to a developing foetus; there is no adequate data in humans to establish safety. Women who could become pregnant are advised to use effective contraception throughout treatment.

Pancreatitis deserves particular attention. In January 2026 the MHRA issued a Drug Safety Update specifically flagging acute pancreatitis as an infrequent but serious known risk with GLP-1 receptor agonists. Anyone with a history of the condition needs an honest conversation with a prescriber before starting, and if severe, persistent abdominal pain develops at any point during treatment, that is a reason to seek urgent medical help rather than wait for a routine review.

Mental health history is handled at prescriber discretion. There is no absolute contraindication, but the Patient Information Leaflets for both medicines note that patients should tell their prescriber if they experience low mood, depression, or thoughts of self-harm. A prescriber may want to discuss this in more detail before approving treatment for someone with relevant history.

Understanding the full picture of how these injections work helps contextualise why some of these cautions exist: slowed gastric emptying, the central mechanism behind appetite reduction, is also what creates the pancreatitis risk signal and the drug-absorption interactions described below.

How contraception, HRT, and other medicines fit into the picture

GLP-1 medicines slow the rate at which food and anything else leaves the stomach. For most medicines this effect is modest. For a few, it matters enough to require a practical adjustment.

For tirzepatide specifically, UK guidance advises that women using oral contraceptives should add a non-oral method (condoms, for example) for the first four weeks of treatment and for four weeks after each dose increase. This is not a reason to avoid treatment; it is a reason to plan ahead. The interaction between weight loss treatment and contraception is covered in more detail on a dedicated page. Current evidence suggests semaglutide carries a lower risk of affecting pill absorption, though the conversation with your prescriber remains important.

HRT is a related consideration. NHS England's guidance on weight management injections notes that transdermal forms of HRT (patches and gels) may be preferable to oral forms for women on tirzepatide, for the same gastric-emptying reason. This isn't a contraindication; it's a sensible adjustment that your prescriber and GP can discuss together.

If you take medicines with a narrow therapeutic window (warfarin is the example most clinicians reach for first) closer monitoring may be warranted when you start treatment, because the change in gastric emptying can affect absorption timing. Insulin and sulfonylureas in people with type 2 diabetes also require dose review, since GLP-1 medicines lower blood sugar independently. The overview of weight loss medication injections covers the broader pharmacology for readers who want more background.

One thing worth knowing: if treatment is approved and you're going through the process with a regulated UK pharmacy, the medicine arrives in plain, unbranded packaging with a DPD tracking notification so you know exactly when to expect it, a practical detail, but it matters to people who care about discretion.

For a full picture of the medicines available and what separates them, the guide to injection medication names is worth reading alongside this page. And if questions about cost are part of your thinking, a factual look at what UK weight loss injections cost privately covers the landscape without any sales pressure.

Under-18s and the groups for whom these medicines are not licensed

Both Mounjaro and Wegovy are licensed for adults only. Neither has sufficient safety and efficacy data in people under 18 to support prescribing, and no regulated UK pharmacy should be supplying them to minors. The lower age limit is a hard line, not a clinical judgement call.

Severe renal impairment and end-stage kidney disease are not absolute contraindications in the formal sense, but they are listed as situations requiring caution, the clinical data in those populations is limited, and a prescriber will want to consider kidney function carefully, particularly since severe dehydration from GI side effects could place additional burden on the kidneys.

People with a history of severe hypersensitivity reactions (anaphylaxis, angioedema) to any component of the pen should not use it. A history of milder injection-site reactions is handled differently, with closer monitoring.

The injection technique guidance is a separate but practical part of safe use for anyone approved for treatment: rotating sites, checking the pen before each dose, and handling the needle correctly all reduce the risk of injection-site reactions from the outset. If you want to understand how a clinician-led service approaches safe prescribing, the about us page sets out how the clinical team and pharmacy are regulated.

If you're unsure whether anything in your history applies here, the clearest next step is a free consultation reviewed by one of our GPhC-registered prescribers, who will go through your medical history in full before any decision is made. Start your free consultation when you're ready.