In short: Tirzepatide (brand name Mounjaro) is a once-weekly prescription-only injection that acts on two gut hormone receptors, GLP-1 and GIP, to reduce appetite and help you feel full sooner. In the UK it is licensed for weight management in adults with a BMI of 30 or more, or 27 to under 30 with a weight-related condition, alongside a reduced-calorie diet and more physical activity. NICE recommends it for some people on the NHS, and it is also prescribed privately after a clinical assessment. A prescriber must decide whether it is suitable for you.

This guide, written by a Ready Health prescribing pharmacist, explains how tirzepatide works, how it is used, its side effects and safety warnings, and what to expect if you stop.

Key takeaways

Tirzepatide is a weekly injection that targets both GLP-1 and GIP receptors to reduce appetite.
Treatment starts at 2.5 mg and is increased gradually; the maximum dose is 15 mg weekly.
Digestive side effects are common, especially after dose increases; pancreatitis is rare but serious.
It may make the contraceptive pill less effective, and it should not be used in pregnancy or breastfeeding.
It works best alongside changes to diet and activity, and weight often returns after stopping.

What is tirzepatide?

Tirzepatide was first licensed for type 2 diabetes and, since late 2023, has also been licensed by the MHRA for weight management. It is the first medicine in the UK to act on both the GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide) receptors, which is why it is sometimes called a "dual agonist".

How does tirzepatide work for weight loss?

GLP-1 and GIP are hormones released by the gut after eating. By mimicking their effects, tirzepatide:

acts on appetite centres in the brain, reducing hunger and food cravings
slows stomach emptying, so you feel full for longer
helps the body release insulin when blood sugar is high and reduces excess glucagon

The overall effect is that most people eat less without feeling as hungry. In the SURMOUNT-1 trial, adults with obesity (without diabetes) who took the highest dose for 72 weeks alongside lifestyle support lost on average around 21% of their body weight, compared with about 3% with placebo. Individual results vary considerably.

How long does tirzepatide take to work?

Many people notice a reduction in appetite within the first few weeks, but weight loss is gradual because the dose is increased slowly. Most of the effect builds over several months. NICE advises that, in the NHS, treatment should be reviewed and stopped if less than 5% of starting weight has been lost after six months on the highest tolerated dose.

Who is tirzepatide licensed for?

The UK licence covers adults with:

a BMI of 30 kg/m² or more (obesity), or
a BMI of 27 to under 30 kg/m² (overweight) with at least one weight-related condition, such as prediabetes or type 2 diabetes, high blood pressure, abnormal cholesterol, obstructive sleep apnoea or cardiovascular disease

Meeting these criteria does not automatically mean tirzepatide is right for you. A prescriber will review your medical history, current medicines and any reasons to avoid it before deciding.

Can I get tirzepatide on the NHS?

Yes, for some people. NICE (TA1026) recommends tirzepatide for managing obesity, and NHS England began a phased rollout through GP practices in June 2025. The first groups prioritised were adults with a BMI of 40 or more (lower thresholds apply for some ethnic backgrounds) and at least four specified weight-related conditions, with eligibility widened in 2026 and further stages planned. Local arrangements vary, so ask your GP whether you qualify. Tirzepatide can also be prescribed privately by a registered prescriber after a consultation.

How is tirzepatide taken?

Tirzepatide is injected under the skin once a week, on the same day each week, at any time of day, with or without food. It can be injected into the tummy, thigh or upper arm, and it is sensible to rotate the site each week. Your prescriber or pharmacist will show you how to use your pen and how to store it; always follow the patient information leaflet.

Stage	Weekly dose	Notes
Starting dose	2.5 mg for 4 weeks	Helps your body adjust; not a long-term dose
Dose increases	5 mg, 7.5 mg, 10 mg, 12.5 mg	Increased in 2.5 mg steps after at least 4 weeks on each dose, only if needed and tolerated
Maintenance	5 mg, 10 mg or 15 mg	15 mg is the maximum dose

What should I do if I miss a dose?

If it is 4 days (96 hours) or less since the missed dose was due, take it as soon as you remember, then continue on your usual day.
If it is more than 4 days, skip the missed dose and take the next one on your usual day.
You can change your injection day if needed, as long as there are at least 3 days (72 hours) between doses.
If you have missed several doses in a row, contact your prescriber before restarting, as you may need a lower dose.

How long will I need to take tirzepatide?

There is no fixed length of treatment. It is a long-term medicine for a long-term condition, and your prescriber should review regularly whether it is still helping and still appropriate. NHS use follows NICE criteria on continuation.

What happens when you stop tirzepatide?

Appetite usually returns over the weeks after stopping, and trials have shown that many people regain a significant amount of the weight they lost. Building habits around eating, activity, sleep and support while on treatment can help, and stopping should be planned with your prescriber, especially if you also take diabetes medicines.

What are the side effects of tirzepatide?

Very common or common side effects (most often when starting or increasing the dose):

nausea, vomiting, diarrhoea and constipation
indigestion, tummy pain, bloating and burping
tiredness, dizziness and injection-site reactions
hair thinning, often linked to rapid weight loss
low blood sugar if you also take insulin or a sulfonylurea

Less common but serious:

Acute pancreatitis - in January 2026 the MHRA strengthened warnings for all GLP-1 based medicines, including tirzepatide. Get urgent medical help for severe, persistent tummy pain that may spread to your back, with or without vomiting. See our guide to upper tummy (epigastric) pain.
Gallbladder problems such as gallstones
Dehydration from vomiting or diarrhoea, which can affect the kidneys
Allergic reactions

Tell any doctor or anaesthetist that you take tirzepatide before surgery or sedation, as slowed stomach emptying can increase the risk of stomach contents entering the lungs. Report suspected side effects to the MHRA Yellow Card scheme.

Who should not use tirzepatide?

People who are pregnant, trying to conceive or breastfeeding. The MHRA advises stopping tirzepatide at least one month before trying for a baby.
Anyone allergic to tirzepatide or any ingredient in the pen.
Children and young people under 18 (not licensed for weight management).

Extra caution, or an alternative, may be needed if you have had pancreatitis, have severe digestive problems such as gastroparesis, have diabetic eye disease, kidney problems, or a history of an eating disorder. This is why a full clinical assessment is essential.

What should I eat while taking tirzepatide?

No foods are strictly banned, but some changes can make side effects easier to manage:

eat smaller meals slowly and stop when you feel full
limit fatty, fried, very sugary or very spicy foods if they trigger nausea or indigestion
prioritise protein, vegetables, fruit and wholegrains to protect muscle and nutrition
drink plenty of fluids, especially if you have diarrhoea or vomiting
keep alcohol within low-risk limits, as it can worsen nausea and affect blood sugar

For more ideas see our articles on healthy eating for lasting weight loss and sustainable weight-loss tips.

Which medicines may interact with tirzepatide?

Oral contraceptive pills: tirzepatide may make them less effective in people living with overweight or obesity. The MHRA advises using a non-oral method (such as a coil or implant) or adding a barrier method for 4 weeks after starting and for 4 weeks after each dose increase.
Insulin and sulfonylureas (such as gliclazide): the dose may need reducing to lower the risk of hypoglycaemia. Read more about type 2 diabetes and insulin resistance.
Medicines that need careful blood-level monitoring, such as warfarin: slowed stomach emptying can affect absorption, so extra monitoring may be needed.
Other GLP-1 based medicines: never use two at the same time.

Always tell your prescriber about all medicines, including over-the-counter and herbal products.

Frequently asked questions

Is tirzepatide the same as semaglutide?

No. Both are weekly injections used for weight management, but semaglutide acts only on GLP-1 receptors, while tirzepatide acts on GLP-1 and GIP receptors. See our article on the head-to-head trial comparing the two.

Can I take tirzepatide during Ramadan?

Many people can, with planning. Read our guide to weight-loss medicines and fasting during Ramadan and speak to your prescriber well beforehand.

Are injections or tablets better for weight loss?

It depends on the person. Several options are now licensed in the UK, including weekly injections and daily tablets. See our comparison of injections and tablets.

What if I am not losing weight?

Some medical conditions and medicines can make weight loss harder. Our article on medical reasons you may struggle to lose weight explains more. Your prescriber can review whether treatment is right for you. If medicines are not suitable, some people may consider weight-loss surgery.

Sources and further reading

Last reviewed: October 2026 by Shamir Patel, Independent Prescribing Pharmacist, Ready Health. This article is general information and does not replace advice from your own clinician. Brand names are trademarks of their respective owners.

Find more articles in our weight-loss blog, or read about weight-loss support in Horwich. If you would like to discuss your options, you can book a weight-management consultation at Ready Health in Standish, Wigan. A prescriber will assess your health and decide with you whether any medicine is suitable.

What Is Tirzepatide and How Does It Work for Weight Loss?