Who Cannot Take Weight Loss Injections — and What to Do Instead

You've been told these injections might work for you, but something on the form gave you pause

You've read about the results, maybe spoken to someone who has used Mounjaro or Wegovy, and you are ready to explore it seriously. Then you hit the health questionnaire and a condition or medication you have gives you a moment of doubt. That hesitation is worth taking seriously. Weight loss injections are not suitable for everyone, and a thorough clinical assessment exists precisely to catch the situations where the risks outweigh the benefits.

The contraindications below are drawn from the licensed product information for both tirzepatide and semaglutide, summarised in plain language. They are not designed to put you off, they are designed to keep the right people safe. Separately, we have written about who these treatments are licensed for, which gives the positive picture alongside this one.

One thing worth saying plainly: a contraindication is not always a firm door closed forever. Some are absolute (pregnancy, MEN2, medullary thyroid cancer). Others depend on severity, how well-controlled a condition is, or what other medicines you take. That distinction matters, and it is exactly what a prescriber is there to work through with you.

Medical conditions that rule out or require very careful prescriber review before starting

A personal or close family history of medullary thyroid carcinoma, or a diagnosis of Multiple Endocrine Neoplasia type 2, is a firm contraindication for both Mounjaro and Wegovy. Animal studies raised concern about thyroid C-cell tumours with this class of medicine; the human relevance is still being studied, but the caution is absolute in the licensed guidance. You cannot start either medicine if this history applies to you.

Pancreatitis is the other high-profile concern. In January 2026 the MHRA issued a Drug Safety Update specifically addressing acute pancreatitis as a known, though infrequent, serious risk with GLP-1 medicines, the Yellow Card reporting scheme continues to monitor cases in the real-world population. Anyone with a current diagnosis of pancreatitis should not take these medicines. A past episode is not automatically a block, but it requires honest disclosure and genuine prescriber judgment.

Severe gastroparesis (a condition where the stomach empties very slowly) is another situation where these medicines are generally not appropriate, since they slow gastric emptying further. Inflammatory bowel disease in an active flare, and other serious gastrointestinal conditions, also call for close prescriber consideration. Severe kidney impairment or a history of diabetic retinopathy may also affect suitability, particularly for semaglutide. The NHS medicines page for tirzepatide lists the conditions to disclose before starting, and is worth reading alongside any consultation.

Life stages and circumstances where weight loss injections are not recommended

Pregnancy is a clear-cut case. Neither Mounjaro nor Wegovy is recommended during pregnancy. Animal studies have shown harmful effects on foetal development at doses relevant to humans, and the clinical trials excluded pregnant women entirely, so there is no safety data to draw on. Anyone who becomes pregnant while using one of these medicines should stop treatment and contact their prescriber or GP promptly.

Breastfeeding is likewise not recommended. It is not known whether tirzepatide or semaglutide passes into breast milk in clinically meaningful amounts, and the conservative position is to avoid both medicines until feeding has stopped. People who are actively trying to conceive are advised not to start treatment. The MHRA guidance recommends using effective contraception throughout treatment; for tirzepatide specifically, oral contraceptive pill users should add a non-oral method for the first four weeks of each new dose, since gastric slowing may reduce absorption. Our guide on who these treatments suit goes deeper on the contraception question.

Under-18s are excluded from both medicines in the UK. Neither is licensed for anyone below 18, and no private prescription can be legitimately issued for a minor. Age is one constraint a prescriber cannot work around.

Medicines that interact, and other practical red flags

These injections are not taken in isolation. A number of medicines interact with tirzepatide and semaglutide in ways that matter clinically. The most practically significant is oral contraceptives: as above, absorption may be reduced during dose escalation with tirzepatide. For people taking insulin or insulin secretagogues (such as sulphonylureas) alongside a GLP-1 medicine for type 2 diabetes, dose adjustments may be needed to manage hypoglycaemia risk, though this is a managed clinical process, not necessarily a reason to avoid treatment entirely.

Oral medicines that depend on precise timing and steady absorption (certain immunosuppressants, thyroid replacements, some anticoagulants) may need monitoring if gastric emptying is slowed. This is a question for your prescriber and, where relevant, your GP. It is also why the consultation process at a regulated pharmacy involves a full medicines review rather than a quick one-question tick-box.

A note on timing: if your circumstances are about to change (you are coming off a medicine in the next few weeks, planning surgery, or considering stopping contraception) it is worth factoring that in before you start. The cost and logistics of pausing treatment mid-titration are real, and planning avoids them. There is no pressure to start this month rather than the next; our prescribers are there to help you choose the right moment, not just the right medicine. If you want to understand the full landscape of weight loss injections available in the UK, that overview is a good starting point before booking a consultation.

For anyone who has worked through all of this and wants a clinician's view of their own situation, you can speak to our prescribers through a free consultation, reviewed the same day by a real clinician, not software. There is no obligation, and if treatment is not right for you, we will say so.