Wegovy and Mounjaro: How do weight loss drugs work?

Medication for weight loss (such as Wegovy and Mounjaro) are used alongside a reduced-calorie diet and increased physical activity.

GLP-1 and GIP are food intake-related incretin hormones that signal to the brain and help to regulate blood sugar levels; these hormones are enhanced by medications such as Wegovy and Mounjaro.

Wegovy and Mounjaro aid:

• to control your appetite by working on the satiety and reward centres in your brain and make you feel fuller by slowing down how quickly your stomach empties
• to release insulin when your blood sugar is high
• assist the regulation of fat utilisation, helping to reduce your cardiovascular risk
•  assist you to safely lose up to 20% body weight and target your metabolism for sustainable weight loss alongside nutrition & exercise plans designed to also maintain and build muscle during weight loss

Wessex Private GP typically uses the following BMI thresholds for prescribing weight loss medications:

BMI of 30 or higher: Generally required for most weight loss medications.

BMI of 27 or higher: If the patient has weight-related health conditions such as type 2 diabetes, hypertension, or sleep apnoea.

These thresholds are in place to ensure that medications are prescribed to those who are most likely to benefit from them.

Avoid in pregnancy and during breast-feeding. It is recommended to use contraception, and special precautions apply when using oral contraceptives.

These medicines may also have direct cardiovascular and neuroprotective effects, potentially offering benefits for brain health, although more research is needed to fully understand these effects.

Hypoglycaemia is not a side effect for either Wegovy or Mounjaro alone but is a potential risk if you’re taking certain diabetes medications, such as insulin, sulphonylureas and glinides which could potentially affect driving and operating machines.

Extra caution is advised for patients with a pre-existing retinopathy, usually associated with diabetes.

While both medications enhance GLP-1, Mounjaro additionally targets another hormone called GIP, which offers more comprehensive benefits compared to Wegovy, which primarily focuses on GLP-1.

Mounjaro is often better tolerated due to its dual action on GLP-1 and GIP, which can result in fewer side effects compared to medications that solely target GLP-1. The addition of GIP helps to balance the effects of GLP-1, potentially reducing common side effects such as nausea and gastrointestinal discomfort. This makes Mounjaro a more comfortable option for many patients, improving their overall treatment experience and adherence to medication.

Mounjaro also encourages your body to use fat stores as a source of energy and is considered a more potent in its effect on satiety (feeling full/satiated) and reward centres (reducing craving).

Side effects of Wegovy and Mounjaro

Common side effects of weight loss medications

Here are the common side effects (may affect 1in 10 people) of these weight loss medications, which don’t tend to be severe and often disappear over time:

• Nausea
• Vomiting
• Diarrhoea
• Heartburn
• Stomach pain
• Constipation
• Headaches
• Fatigue
• Injection site reaction (itching, redness, pain)

Uncommon side effects of Wegovy and Mounjaro

These may affect 1 in 100 people:

• Hypoglycaemia
• Cholecystitis (infection of gallbladder)
• Change of taste, change in skin sensation

Severe allergic reactions, such as breathing problems, rapid swelling of lips, tongue and/or throat) are rare (may affect 1in1000 people); you need to get immediate help.

All side effects can vary in severity and do not affect everyone. Please read the PIL for a full list.

Long-term risks of weight loss medications

There are some potential long-term risks associated with weight loss medications, such as:

• Persistent nausea, vomiting, diarrhoea, and constipation.
• Increased risk of pancreatitis (inflammation of the pancreas), which is uncommon and may affect 1in100 people.
• Higher likelihood of gallstones and gallbladder disease.
• Potential for increased heart rate and blood pressure.
• Risk of kidney damage or worsening of existing kidney conditions.
• Possible mood changes, including depression and anxiety.

Mounjaro may rarely cause tumours in the thyroid, including thyroid cancer. Watch for possible symptoms, such as a lump or swelling in the neck, hoarseness, trouble swallowing, or shortness of breath. If you have any of these symptoms, tell your doctor.

Do not use Mounjaro if you or any of your family have ever had a type of thyroid cancer called MTC or if you have an endocrine system condition called MEN 2, or if you have had a serious allergic reaction to tirzepatide or any of the ingredients in Mounjaro.

Potential benefits of weight loss medications

Reduced Cancer Risk: Weight loss itself can lower the risk of several obesity-related cancers, such as breast, endometrial, and colorectal cancers. By helping with weight loss, these medications may indirectly reduce cancer risk.

Please fully disclose your personal and family medical history so that we can together weigh up the benefits and risks of these medications for your specific situation.

Mounjaro is a new drug, it is subject to additional monitoring to quickly identify new safety information. Regular monitoring is essential to ensure safety and effectiveness.

Suggested length of treatment

Mounjaro is considered safe for longer-term use and most effective when used this way and should be taken until your weight loss goals have been reached. Research shows that Mounjaro reaches its prime efficacy between 18 and 24 months.

Please see the table below for guidance:

Coming off weight loss drugs

You should wean off the medication slowly while focusing on developing healthy habits and consider a washout period if switching between medications.

After a longer break of three weeks or more you should not restart at your previous dose. Expect to start at a lower dose and gradually work back up.

Furthermore, the benefits of treatment if less than 5% of the initial body weight is lost after 6 months at the highest tolerated dose, require a thorough reappraisal.

Reviews on the independant pharmacy website indicate that the majority of people are happy with their use of either Mounjaro or Wegovy. Overall, Mounjaro gets a score of 4.7 out of 5, while Wegovy is given a slightly lower score of 4.48.

Mounjaro and Wegovy work in slightly different ways and can both be very effective weight loss treatments; studies underscore the importance of lifestyle changes and ongoing medical supervision to maximise the efficacy of these weight loss drugs.

For more advice, book an initial consultation at our Weight Loss Clinic.