Absolute contraindications: who should never take weight-loss injections
Certain conditions mean weight-loss injections are not safe for you under any circumstances. These are called absolute contraindications.
Personal or family history of medullary thyroid cancer
If you or a close family member (parent, sibling, or child) has ever had medullary thyroid carcinoma (MTC), you should not take Mounjaro, Wegovy, or any other GLP-1 medication.1
In animal studies, tirzepatide and semaglutide caused thyroid C-cell tumours in rats at clinically relevant doses.1 While it’s unknown whether this occurs in humans, the potential risk means these medications carry a boxed warning, the strongest safety warning required by regulators.
If you notice any symptoms of thyroid tumours while taking these medications – such as a lump in your neck, difficulty swallowing, shortness of breath, or persistent hoarseness – contact your doctor immediately.
Multiple Endocrine Neoplasia syndrome type 2 (MEN 2)
MEN 2 is a rare genetic condition that significantly increases the risk of developing thyroid tumours. If you have this condition, weight-loss injections are not suitable for you.1
Previous serious allergic reaction
If you’ve ever had a severe allergic reaction to tirzepatide, semaglutide, liraglutide, or any ingredients in these medications, you should not take them again.1
Serious allergic reactions can include anaphylaxis and angioedema (severe swelling). If you’ve experienced these with any GLP-1 medication in the past, your prescriber should not prescribe another one.
Type 1 diabetes or diabetic ketoacidosis
Weight-loss injections are not approved for people with type 1 diabetes. These medications work by stimulating insulin release in response to food, which requires functioning pancreatic beta cells, something people with type 1 diabetes don’t have.3
If you have a history of diabetic ketoacidosis (DKA), these medications are also contraindicated.3
Medical conditions requiring extra caution
Some conditions don’t completely rule out weight-loss injections, but they do require careful consideration and closer monitoring.
History of pancreatitis
Pancreatitis (inflammation of the pancreas) has been reported in people taking GLP-1 medications.1 If you have a history of pancreatitis, your prescriber needs to weigh the benefits against the risks carefully.
You should stop taking weight-loss injections immediately and contact your doctor if you experience severe pain in your upper abdomen that radiates to your back, along with nausea and vomiting. These are warning signs of pancreatitis.
Severe gastroparesis
Gastroparesis is a condition where the stomach empties food more slowly than usual. Since GLP-1 medications already slow gastric emptying as part of how they work, they’re not recommended for people with severe gastroparesis.4
If you have mild gastroparesis, your prescriber may still consider treatment but will monitor you closely for worsening symptoms.
Gallbladder disease
A meta-analysis of 76 randomised controlled trials found that GLP-1 medications are associated with a 37% increased risk of gallbladder or biliary diseases, including gallstones and cholecystitis.5 This risk appears higher with higher doses and longer treatment duration, and may be partly related to rapid weight loss itself.
If you’ve had gallbladder problems in the past, your prescriber should monitor you for symptoms like pain in your upper right abdomen, fever, jaundice (yellowing of skin or eyes), or clay-coloured stools.
Having your gallbladder removed (cholecystectomy) doesn’t prevent you from taking these medications, and in fact, it may reduce certain risks since gallstones can no longer form.5
Kidney disease
GLP-1 medications can cause dehydration through their gastrointestinal side effects (nausea, vomiting, diarrhoea), which can worsen kidney function.1
If you have existing kidney disease, you may still be able to take weight-loss injections, but you’ll need more frequent monitoring of your kidney function.
It’s essential to stay well hydrated, especially during the early weeks of treatment when side effects are most common.
Diabetic retinopathy
If you have diabetes and diabetic retinopathy (damage to the blood vessels in your eyes), rapid improvements in blood sugar control can sometimes temporarily worsen eye problems.1
Your prescriber should be aware of any existing eye conditions before starting treatment, and you should report any vision changes promptly.
History of depression or mental health conditions
The MHRA has reviewed the evidence and found no causal link between GLP-1 medications and suicidal behaviour, self-injury, or depression.6 However, your prescriber should be aware of your mental health history.
If you experience any changes in mood, thoughts of self-harm, or worsening depression while taking weight-loss injections, contact your healthcare team.
Drug interactions to be aware of
Weight-loss injections can interact with other medications. Always tell your prescriber about everything you’re taking, including over-the-counter medicines and supplements.
Other GLP-1 medications
You should never take more than one GLP-1 medication at the same time.7 This includes:
- Ozempic (semaglutide)
- Saxenda (liraglutide)
- Victoza (liraglutide)
- Trulicity (dulaglutide)
- Byetta or Bydureon (exenatide)
Combining these medications increases the risk of serious side effects without additional benefit.
Insulin and sulfonylureas
If you’re taking insulin or sulfonylurea tablets (such as gliclazide or glimepiride) for diabetes, adding a weight-loss injection significantly increases your risk of low blood sugar (hypoglycaemia).7
Your prescriber will likely need to reduce your insulin or sulfonylurea dose when starting Mounjaro or Wegovy. You may also need to monitor your blood sugar more frequently.
DPP-4 inhibitors
DPP-4 inhibitors (medications ending in ‘-gliptin’, such as sitagliptin or linagliptin) act through pathways similar to those of GLP-1 medications. Taking them together is not recommended.7
If you’re currently taking a DPP-4 inhibitor, your prescriber should switch you to an alternative medication before starting weight-loss injections.
Oral contraceptives (with tirzepatide only)
Tirzepatide (Mounjaro) can reduce the effectiveness of oral contraceptive pills.8 If you’re taking the combined pill or progestogen-only pill, you should either:
- Switch to a non-oral contraceptive method (such as an implant, injection, or IUD), or
- Use additional barrier contraception (such as condoms) for 4 weeks after starting tirzepatide and for 4 weeks after each dose increase
This interaction does not apply to semaglutide (Wegovy) or to contraceptives that aren’t taken orally (patches, rings, implants, injections, or IUDs).8
Warfarin and other blood thinners
If you take warfarin or other coumarin anticoagulants, your INR (blood clotting time) should be monitored more frequently when starting or adjusting your weight-loss injection dose.7
Medications requiring precise absorption timing
Because GLP-1 medications slow gastric emptying, they can affect the rate of absorption of other oral medications.9 This is generally not a significant problem, but if you take any medication where precise timing of absorption is important (such as some antibiotics that need to be taken on an empty stomach), discuss this with your prescriber or pharmacist.
Age and pregnancy considerations
Pregnancy and breastfeeding
Weight-loss injections should not be used during pregnancy. Animal studies have shown potential risks to the developing foetus, and the effects on human pregnancy are not fully known.1
If you’re planning to become pregnant, you should stop taking weight-loss injections at least 2 months before trying to conceive. This allows the medication to clear from your system.1
If you become pregnant while taking these medications, stop immediately and contact your doctor.
It’s also not recommended to take weight-loss injections while breastfeeding, as it’s unknown whether the medication passes into breast milk.
Under 18 years old
Wegovy is licensed for use in adolescents aged 12 and over with obesity in the UK.2 Mounjaro is currently only licensed for adults aged 18 and over.
If you’re under 18, any decision about weight-loss medications should involve a specialist and your parents or guardians.
Adults over 65
Weight-loss injections can be used by older adults, but there are some additional considerations.10
Older adults may be more sensitive to side effects, particularly:
- Dehydration: The gastrointestinal side effects (nausea, vomiting, diarrhoea) can cause dehydration, which is more dangerous in older people and can worsen kidney function or cause dizziness and falls
- Muscle loss: Weight loss always involves some loss of muscle mass, and older adults are already at risk of age-related muscle loss (sarcopenia). Eating adequate protein and doing strength exercises is particularly important
- Drug interactions: Older adults often take more medications, increasing the risk of interactions
There is very limited data on using these medications in people over 85, so prescribers should be particularly cautious in this age group.3
Warning signs that mean you should stop and seek help
While most side effects of weight-loss injections are mild and manageable, some symptoms require urgent attention. Stop taking your medication and contact your prescriber or seek medical help if you experience:
Signs of severe allergic reaction
- Difficulty breathing or swallowing
- Severe swelling of the face, lips, tongue, or throat
- Severe rash or itching
- Rapid heartbeat with feeling faint
Signs of pancreatitis
- Severe pain in your upper abdomen that radiates to your back
- Pain that doesn’t go away and may worsen after eating
- Severe nausea and vomiting
Signs of gallbladder problems
- Pain in the upper right side of your abdomen
- Fever
- Yellowing of skin or eyes (jaundice)
- Clay-coloured stools
Signs of severe dehydration
- Unable to keep fluids down for more than 24 hours
- Very dark urine or not passing urine
- Severe dizziness or confusion
- Rapid heartbeat
Signs of kidney problems
- Decreased urination
- Swelling in legs or ankles
- Feeling very tired
Before surgery or medical procedures
If you’re scheduled for surgery or any procedure requiring general anaesthesia or deep sedation, tell your surgical team that you’re taking a weight-loss injection.6
These medications slow stomach emptying, which increases the risk of food or liquid entering your lungs during anaesthesia. Your surgical team may give you specific fasting instructions or modify their anaesthetic technique.
Questions to ask your GP before starting weight-loss injections
Before starting treatment, make sure you’ve discussed these points with your prescriber:
About your medical history:
- Do any of the absolute contraindications apply to me?
- Given my health conditions, am I a suitable candidate for this medication?
- What additional monitoring will I need?
About your current medications:
- Do any of my current medications interact with weight-loss injections?
- Will any of my medication doses need to be adjusted?
- Do I need to change my contraception method?
About managing side effects:
- What side effects should I expect, and how can I manage them?
- What warning signs should prompt me to contact you?
- How much weight loss is too rapid?
About follow-up and monitoring:
- How often will I need check-ups?
- What tests will you do to monitor my health?
- Who should I contact if I have concerns between appointments?
Frequently asked questions
Can I take weight-loss injections if I’ve had my gallbladder removed?
Yes. Having your gallbladder removed is not a contraindication to taking Mounjaro or Wegovy.5 However, you should tell your prescriber about your surgical history, and they may monitor you for any digestive issues.
Are weight-loss injections safe if I have high blood pressure?
In most cases, yes. Weight-loss injections often improve blood pressure as you lose weight. However, your prescriber should monitor your blood pressure, and you may need adjustments to any blood pressure medications you’re taking.
Can I take weight-loss injections if I have PCOS?
Yes. Weight-loss injections are often beneficial for people with polycystic ovary syndrome (PCOS), as weight loss can improve hormonal balance and symptoms.
However, if you’re using oral contraceptives and taking tirzepatide (Mounjaro), you’ll need additional contraception or an alternative method.
What BMI do I need to be prescribed weight-loss injections in the UK?
For private prescriptions, you typically need a BMI of 30 or above, or a BMI of 27 or above with at least one weight-related health condition (such as type 2 diabetes, high blood pressure, or sleep apnoea).2
NHS access criteria vary by region, but NICE guidance allows tirzepatide for people with a BMI of 35 or above with weight-related comorbidities, or a BMI of 30 or above for people from South Asian, Chinese, Black African, African-Caribbean, or Middle Eastern backgrounds.2
Can weight-loss injections cause cancer?
In animal studies, GLP-1 medications caused thyroid C-cell tumours in rats.1 However, it’s unknown whether this occurs in humans. This is why these medications are contraindicated for anyone with a personal or family history of medullary thyroid cancer or MEN 2.
Large-scale human studies have not shown an increased risk of other cancers with these medications.
Are weight-loss injections addictive?
No. Weight-loss injections are not addictive. They work on appetite hormones, not on reward pathways in the brain like addictive substances.
You can stop taking them at any time, although your appetite will return to normal, which is why developing healthy habits while on medication is important.
Can I drink alcohol while taking weight-loss injections?
There’s no direct interaction between alcohol and GLP-1 medications.7 However, alcohol can worsen nausea and digestive side effects, and it adds empty calories that may slow your progress. If you have diabetes, alcohol can also affect blood sugar levels.
Do I need to take weight-loss injections forever?
This depends on your individual situation. Some people use weight-loss injections as a tool to help them develop healthier habits and then gradually stop.
Research shows that people who stop GLP-1 medications without behaviour change support often regain weight.11
At Second Nature, we focus on helping you build sustainable habits while on medication, so you have the skills to maintain your weight loss if you choose to stop.
Take home message
Weight-loss injections like Mounjaro and Wegovy are effective treatments for people living with obesity, but they’re not suitable for everyone.
Absolute contraindications include a personal or family history of medullary thyroid cancer, MEN 2, previous serious allergic reactions to these medications, type 1 diabetes, and diabetic ketoacidosis.
Conditions requiring careful consideration include a history of pancreatitis, severe gastroparesis, gallbladder disease, kidney disease, and diabetic retinopathy.
If you’re currently taking insulin, sulfonylureas, DPP-4 inhibitors, or oral contraceptives (with tirzepatide), your prescriber will need to adjust your treatment plan.
The most important thing is to have an open conversation with your prescriber about your complete medical history, current medications, and any concerns.
They can help you decide whether weight-loss injections are appropriate for you and create a monitoring plan to keep you safe.
Second Nature's Mounjaro and Wegovy programmes
Second Nature provides Mounjaro or Wegovy as part of our Mounjaro and Wegovy weight-loss programmes.
Why choose Second Nature over other medication providers, assuming you're eligible?
Because peace of mind matters.
We've had the privilege of working with the NHS for over eight years, helping people across the UK take meaningful steps toward a healthier, happier life.
Our programmes are designed to meet people where they are, whether that means support with weight loss through compassionate one-to-one health coaching, or access to the latest weight-loss medications (like Mounjaro and Wegovy) delivered alongside expert care from a multidisciplinary team of doctors, psychologists, dietitians, and personal trainers.
At the heart of everything we do is a simple belief: real, lasting change comes from building better habits, not relying on quick fixes. We're here to support that change every step of the way.
With over a decade of experience, thousands of lives changed, and a long-standing record of delivering programmes used by the NHS, we believe we're the UK's most trusted weight-loss programme.
We hope to offer you something invaluable: peace of mind, and the support you need to take that first step.
References
- U.S. Food and Drug Administration. (2025). Mounjaro (tirzepatide) prescribing information.
- National Institute for Health and Care Excellence. (2024). Tirzepatide for managing overweight and obesity (TA1026).
- Buckinghamshire, Oxfordshire and Berkshire West ICB. (2024). Tirzepatide (Mounjaro) Initiation Guide.
- Novo Nordisk. (2025). Wegovy side effects information.
- He, L., et al. (2022). Association of glucagon-like peptide-1 receptor agonist use with risk of gallbladder and biliary diseases: a systematic review and meta-analysis. JAMA Internal Medicine, 182(5), 513-519.
- Medicines and Healthcare products Regulatory Agency. (2025). GLP-1 medicines for weight loss and diabetes: what you need to know.
- Specialist Pharmacy Service. (2025). Considerations and interactions with GLP-1 receptor agonists.
- Faculty of Sexual and Reproductive Healthcare. Tirzepatide and oral contraceptive guidance. Referenced in SPS NHS guidance.
- Ceriello, T.D., et al. (2024). Drug-drug interactions between glucagon-like peptide 1 receptor agonists and oral medications: a systematic review. Diabetes, Obesity and Metabolism.
- Marolt, U., et al. (2024). Pharmacological treatment of obesity in older adults. Drugs & Aging, 41(11), 869-891.
- Richards, R., et al. (2025). A remotely delivered, semaglutide-supported weight management programme: 12-month outcomes. JMIR Formative Research.
