GLP-1 Cons: The Honest Downsides (October 2026)

01 · The short answer

What are the negatives of GLP-1 medicines?

The negatives fall into five groups: side effects, the injection routine, cost, variable results and suitability. Side effects are the one most people ask about. The full list, with how common each effect is, sits in the patient information leaflet and the SmPC¹¹³. The most common effects, affecting more than 1 in 10 people, are nausea, diarrhoea, vomiting, constipation and abdominal discomfort. Side effects are most likely when starting treatment or moving up a dose, and for most people they ease within days to a few weeks as the body adjusts. Raise anything new or lasting with your prescriber.

Mounjaro is a once-weekly injection¹. Some people dislike self-injecting. Others find the weekly routine easy. For a side-by-side view, see our guide to the pros and cons of weight loss injections, or the GLP-1 pros for the other side of the ledger.

02 · Your decision

Is taking a GLP-1 worth the risk?

That depends on your health, your goals and your tolerance for side effects, and a prescriber is the right person to weigh it with you. In the SURMOUNT-1 trial, people on tirzepatide lost far more weight on average than those using lifestyle changes alone². These are trial averages — individual results vary, and treatment works alongside a reduced-calorie diet and increased physical activity, not instead of them.²

If you are ready to explore it, our weight loss clinic starts with a free consultation reviewed by a GPhC-registered prescriber. Unsuitable orders are refunded automatically.

Related guides: Mounjaro Bundles UK: Pros and Cons and Cons of mounjaro and Cons of taking mounjaro.

03 · Who should not use it

Who should not use GLP-1 medicines?

Suitability is decided case by case, and the contraindications are set out in the SmPC and patient leaflet¹¹³. Mounjaro is not suitable for everyone. You should not take it if you are allergic to tirzepatide or any of its ingredients, and it should not be used during pregnancy or while trying to conceive. Your prescriber will take extra care, or may recommend against treatment, if you have a history of pancreatitis, severe gut or digestive disease, diabetic retinopathy, or gallbladder problems. It is prescription-only. Mounjaro must not be used during pregnancy, and it should be stopped at least one month before trying to conceive. If you become pregnant during treatment, stop the medicine and tell your prescriber promptly. Two interactions also deserve a mention. It can reduce absorption of oral contraceptives, because stomach emptying slows⁷. The SmPC advises switching to a non-oral method, or adding a barrier method (such as condoms), for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase.¹ Doses of insulin or sulfonylureas may also need adjusting because of the risk of low blood sugar¹. Tell your prescriber about every medicine you take.

04 · Media myths

'Ozempic breasts', 'Ozempic teeth' and smell

These are media phrases, not terms used in the Mounjaro patient leaflet. Ozempic is semaglutide licensed for type 2 diabetes. It is not a UK weight-loss medicine, and a regulated UK pharmacy will not supply it for weight loss. If you notice a new symptom, discuss it with your prescriber and report it through the Yellow Card scheme¹⁹.

05 · Serious side effects

Serious side effects: when to get help straight away

Serious side effects are uncommon but need urgent attention. Seek immediate medical help for: severe, persistent stomach pain that may spread to your back, with or without vomiting (possible pancreatitis); pain in the upper right abdomen, fever or yellowing of skin/eyes (possible gallbladder problems); or signs of a severe allergic reaction such as swelling of the face or throat and difficulty breathing. If you take insulin or a sulfonylurea alongside Mounjaro, watch for low blood sugar (shakiness, sweating, confusion) — your prescriber may need to adjust those medicines.¹

06 · NHS and alternatives

NHS access as a limitation

Mounjaro is available on the NHS, but access is being phased in over several years. NICE recommends it for adults with a BMI of at least 35 plus at least one weight-related condition, and the NHS primary-care rollout began in June 2025 with the highest-need groups first (initially BMI 40+ with four or more qualifying conditions).⁴⁶ Full rollout is expected to take years, which is why many people who meet the licensed criteria choose regulated private treatment.