What is Tirzepatide? How It Works, Side Effects & What to Expect

What is tirzepatide and how does tirzepatide work?

Tirzepatide (pronounced tir-zepa-tide) is a prescription medicine used to treat obesity and type 2 diabetes. It is a type of medication known as a dual agonist, meaning it mimics the actions of two hormones in your body: GIP and GLP-1 [2].

Tirzepatide is sold under the brand name Mounjaro in the UK.

What is a dual agonist?

A dual agonist is a drug that stimulates two hormone receptors at the same time, increasing their effects. In the case of tirzepatide, these hormones are glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). Both GIP and GLP-1 are naturally occurring hormones in your body that help regulate blood sugar, reduce your appetite and help you feel full for longer after eating [2].

Combining both GIP and GLP-1 agonists means dual agonists are more effective for weight loss and blood sugar control than single receptor agonists such as semaglutide [5,6].

How effective is tirzepatide for weight loss?

Studies have shown that tirzepatide is a highly effective medication for weight loss. The main clinical trial to assess the efficacy and safety of tirzepatide is the SURMOUNT-1 trial that was published in the New England Journal of Medicine in 2022 [1].

This clinical trial included 2539 adults with a body mass index (BMI) of 30 or above, or 27 or above and at least one weight-related condition (excluding type 2 diabetes) [1].

During the trial, participants received weekly tirzepatide injections at doses of 5mg, 10mg, or 15mg, or a placebo (fake medicine) for 72 weeks.

The results:

The results of the SURMOUNT-1 trial are as follows: [1]

• On the 5mg dose, participants lost an average of 15 to 16% of their starting body weight
• On the 10mg dose, participants lost an average of 19.5% to 21.4% of their starting body weight
• On the 15mg dose, participants lost an average of 20.9% to 22.5% of their starting body weight
• Participants taking the placebo lost an average of 3.1% of their starting body weight

Other findings included:

• Up to 91% of participants lost at least 5% of their starting body weight, compared to 35% taking the placebo
• Up to 57% of participants taking the 10mg and 15mg doses achieved weight loss of at least 20%, compared to 3% taking the placebo
• The trial also found reductions in blood pressure and waist circumference, which are known risk factors for cardiovascular disease
• Many participants reported an improvement in physical function, emotional, social and psychological well-being and quality of life
• Side effects were generally mild and included nausea, vomiting, diarrhoea and constipation. Only around 4 to 7% of participants left the trial due to side effects.

Who can take tirzepatide?

Whether you are eligible to take tirzepatide for weight loss in the UK depends on whether you get it on the NHS or privately.

To get an NHS prescription for tirzepatide, you normally need a GP referral to a weight management service, and you must meet the following criteria: [3,4]

• A BMI of 40 or over, or 35 and over with one weight-related health condition, such as type 2 diabetes, high blood pressure, sleep apnoea, high cholesterol, or cardiovascular disease.
• People from some ethnic backgrounds, including South Asian, Chinese, other Asian, Middle Eastern, Black African, or African-Caribbean, may be eligible for tirzepatide with a lower BMI.

If you do not meet the NHS criteria, you may be able to get tirzepatide privately. Always use a regulated pharmacy, like we are here at Pharmacy Online (GPhC: 2064448).

Who can’t take tirzepatide?

Tirzepatide isn’t suitable for everyone. Tell your doctor or pharmacist about any medical conditions or allergies you have and any medicines you take regularly, including over-the-counter medicines and supplements.

Tirzepatide is not safe for you if:

• You have Multiple Endocrine Neoplasia syndrome type 2 (MEN2)
• You, or a close relative, has a history of medullary thyroid carcinoma (MTC)
• You are allergic to tirzepatide or any of its ingredients
• You are breastfeeding
• You are pregnant. If you are planning to become pregnant, stop tirzepatide at least one month before trying to conceive. Speak to your prescriber immediately if you become pregnant while taking tirzepatide [2].

Tirzepatide may not be safe for you if you have:

• A history of pancreatitis (inflammation of the pancreas)
• Diabetic retinopathy (eye damage caused by diabetes) or macular oedema (fluid build-up at the back of the eye)
• Some types of severe digestive disease, such as gastroparesis (a condition when the stomach empties too slowly)
• Problems with your heart, liver or kidneys
• A history of eating disorders

How do you take tirzepatide?

You take tirzepatide as a subcutaneous (below the skin) injection once a week [2].

Your healthcare provider will show you how to give your tirzepatide injection correctly. Instructions are also in the patient information leaflet that comes with your medicine. Ask your doctor or pharmacist if you have any questions.

• Take your tirzepatide injection on the same day of the week.
• You can take it at any time of the day, with or without food.
• You can change the day of the week you give your tirzepatide as long as there are at least three days between doses.
• Give your injection in your abdomen (at least 2 inches away from your belly button), thigh, or upper arm. Rotate the injection site each week.
• Wash and dry your hands thoroughly before giving your injection
• Clean the area you are injecting with an alcohol swab before giving your injection. Do not inject into sore, broken or damaged skin.
• To give your tirzepatide injection, pinch the skin, insert your needle at a 90-degree angle and push the plunger down slowly.

Important: Tirzepatide must be stored in a refrigerator between 2°C and 8°C before use. Once removed from the fridge, it can be kept at room temperature (below 30°C) for up to 30 days. Do not freeze. Keep out of direct sunlight. See the patient information leaflet for full storage guidance [2].

Where to inject tirzepatide

As touched on above, you can inject tirzepatide into three areas of your body [2].

• Abdomen (stomach) - inject into the fatty area of your stomach, at least 5 cm (2 inches) away from your belly button
• Upper thigh - inject into the front or outer area of either thigh
• Upper arm - inject into the outer, fatty area of your upper arm

Rotating injection sites:

Do not inject into the same spot each time. Rotate your injection site each week - for example, use your abdomen one week and your thigh the next. Within each site, vary the exact spot slightly to avoid irritating the same area repeatedly.

Sites to avoid:

Do not inject into skin that is sore, bruised, red, swollen, hardened, or broken. Avoid injecting near scars or stretch marks where possible.

If you are unsure about your technique or which site to use, ask your pharmacist or prescriber to talk you through it before your first injection.

Disposing of your used pens safely

Used Mounjaro pens contain a needle and must be disposed of in an approved sharps container - not in your household bin or recycling [11].

You can get a sharps bin on prescription from your GP or pharmacist at no cost. Once your sharps bin is full, contact your local council to arrange a collection, or return it to a pharmacy that accepts sharps for disposal.

Tirzepatide dosage

Tirzepatide is given on a dosing schedule that is increased gradually over several weeks.

The starting dose of tirzepatide is always 2.5mg. It is then increased by 2.5mg, usually every 4 weeks, until you reach your maintenance dose.

Your maintenance dose is the dose you’ll take for the rest of your treatment. This may be 5mg, 10mg or 15mg, depending on your weight loss results and whether you experience side effects.

The maximum maintenance dose of tirzepatide is 15mg per week [2].

What to do if you miss a dose:

If you miss your weekly tirzepatide injection, what you should do depends on how much time has passed since your missed dose [2].

• If it has been 4 days (96 hours) or fewer since your missed dose, take your injection as soon as you remember. Then continue with your usual weekly schedule.
• If more than 4 days have passed, skip the missed dose entirely. Take your next injection on your regularly scheduled day as normal.

Do not take two doses in the same week to make up for a missed one. If you are unsure what to do, or if you have missed more than one dose, speak to your pharmacist or prescriber before taking your next injection.

What happens if you stop Tirzepatide?

If you stop taking tirzepatide, it is likely that some or all of the weight you have lost will return over time. This is not a failure - it reflects the fact that obesity is a long-term condition, and tirzepatide works by actively suppressing appetite and altering how your body processes food. When the medicine is stopped, those effects stop too.

What the evidence shows

The SURMOUNT-4 trial looked specifically at what happens when tirzepatide is discontinued. Participants who stopped tirzepatide after 36 weeks of treatment regained an average of 14% of their body weight over the following year, while those who continued lost a further 6.7%. Most people who stopped regained more than a quarter of the weight they had previously lost [8].

Alongside the weight regain, many of the wider health improvements seen with tirzepatide - including reductions in blood pressure, cholesterol, and blood sugar - also reversed in proportion to the amount of weight regained [9].

Importantly, people who stopped tirzepatide were still, on average, lighter at the end of the study than when they first started treatment. Stopping does not undo everything, but sustaining results does appear to require continued treatment.

Tirzepatide side effects

Like all medicines, tirzepatide can cause side effects in some people. Most side effects are mild and improve as your body adjusts to the medication. If you have side effects that are bothering you, getting worse, or not getting better, make an appointment with your GP.

Common side effects of tirzepatide include:

• Nausea (feeling sick)
• Vomiting (being sick)
• Diarrhoea
• Constipation
• Bloating
• Indigestion and acid reflux
• Fatigue
• Feeling faint or dizzy
• Hair loss
• Injection site reactions (redness, swelling, bruising or itching at the injection site)

Rare, but serious side effects of tirzepatide include:

• Severe allergic reaction (anaphylaxis)
• Acute pancreatitis
• Gallbladder problems
• Kidney problems
• Hypoglycaemia (low blood sugar)

In January 2026, the MHRA strengthened warnings about the risk of severe and fatal pancreatitis with GLP-1 and GIP/GLP-1 medicines, including tirzepatide. If you experience severe abdominal pain that does not go away, seek urgent medical attention. [10]

▼ This medicine is subject to additional monitoring. This will allow quick identification of new safety information. You can help by reporting any side effects you may get. See how to report side effects.

Tirzepatide and other medications

Tell your doctor or pharmacist about all medicines you take before starting tirzepatide, including prescription medicines, over-the-counter medicines, vitamins, and herbal supplements. This is because tirzepatide can affect how some medicines are absorbed by your body: [2]

• Medicines that are absorbed by mouth: Tirzepatide slows down how quickly your stomach empties, which can affect how quickly your body absorbs medicines you take by mouth. This does not mean you cannot take oral medications with tirzepatide, but your doctor may want to monitor you more closely or adjust your doses.
• Oral contraceptives (the pill): if you take an oral contraceptive (the pill), tirzepatide may reduce how well it is absorbed, particularly when you first start treatment. For four weeks after starting tirzepatide, and for four weeks after each dose increase, you should use an additional method of contraception, such as condoms, to ensure you are protected against pregnancy. Alternatively, you may wish to switch to a non-oral method of contraception (such as a patch, implant, or coil) while taking tirzepatide. Speak to your GP or pharmacist for advice on the best option for you.
• Blood thinners and heart medicines: If you take warfarin (a blood thinner) or digoxin (a heart medicine), tirzepatide may affect how quickly these medicines are absorbed by your body. Your doctor or pharmacist will monitor you more closely when you start tirzepatide or after a dose increase. Do not stop taking these medicines without medical advice.
• Insulin and diabetes medicines: If you take insulin or certain diabetes medications, taking tirzepatide at the same time increases your risk of hypoglycaemia (low blood sugar). Your doctor may reduce your insulin dose when you start tirzepatide. Do not adjust your doses yourself without speaking to your doctor first.
• Other weight loss injections: Tirzepatide should not normally be prescribed alongside other medicines in the same class, such as semaglutide (Wegovy or Ozempic) or liraglutide. Your prescriber will ensure this is not the case but let them know if you have recently taken or stopped any other weight loss injections.

If you are unsure whether any of your medicines are affected, ask your pharmacist before starting tirzepatide.

How does tirzepatide compare to other weight loss drugs?

With several weight loss medicines available, it can be difficult to know which one is best for you.

Talking to a healthcare professional, such as your GP, practice nurse, or pharmacist, can help you decide which weight loss medicine may be most suitable for you.

Some things to consider when choosing a weight loss medication include:

• Your general health
• Your medical and family history
• Any other medicines you are taking
• Your preferred method of administration (tablets or injections)
• Whether you experience side effects with a particular medicine
• The cost of the medicine

Tirzepatide vs Semaglutide

Tirzepatide and semaglutide are both highly effective medicines for weight loss, but there are some differences between the two, including the way they work (the mechanism of action), the average expected weight loss and the cost.

Tirzepatide vs Retatrutide

The latest weight loss drug, retatrutide, differs from other weight loss medicines as it targets three hormone receptors: GLP-1, GIP, and glucagon, instead of one or two, making it potentially more effective [12].

Retatrutide is still undergoing clinical trials and isn’t yet approved to be used as a weight loss treatment.

• Read our full guide on Retatrutide

Tirzepatide and lifestyle

Tirzepatide is licensed as an adjunct to a reduced-calorie diet and increased physical activity - meaning it is designed to work alongside lifestyle changes, not instead of them. Clinical evidence supports this. In the SURMOUNT-1 trial, all participants received dietary and physical activity support alongside tirzepatide, and the combination produced substantially greater weight loss than lifestyle changes alone [1].

Making sustainable changes to your diet and activity levels will help you get the most from your treatment and maintain results over the long term.

Diet

You do not need to follow a specific named diet while taking tirzepatide, but a reduced-calorie approach is part of the licensed treatment. In practice, this means eating slightly less than you currently do, with a focus on nutritious, balanced meals. Because tirzepatide reduces appetite, slows stomach emptying, and increases feelings of fullness, many people find it easier to eat smaller portions and feel satisfied with less food than before [2].

The following are general tips that many people find helpful - they are not specific to tirzepatide:

• Prioritise protein at each meal to help preserve muscle strength and keep you feeling fuller for longer
• Eat regular meals rather than skipping them, as irregular eating can increase nausea, particularly during the dose escalation period
• Stay well hydrated - dehydration can worsen side effects and is a particular risk if you experience nausea, vomiting, or diarrhoea
• Avoid large, high-fat meals, particularly in the early weeks of treatment, as these can worsen gastrointestinal side effects such as nausea and bloating

If you are unsure how to adjust your diet, ask your GP or weight management service for a referral to a dietitian.

Excercise

Increased physical activity is a core part of tirzepatide treatment [2]. Aim to build regular movement into your routine - both aerobic exercise (such as walking, swimming, or cycling) and resistance training (such as weights or bodyweight exercises).

As with any significant weight loss, some reduction in muscle mass can occur alongside fat loss. Research from the SURMOUNT-1 trial found that the proportion of muscle to fat lost with tirzepatide was similar to that seen with calorie restriction alone [7]- but because tirzepatide produces greater total weight loss, the absolute amount of muscle lost can be greater. Incorporating resistance exercise helps preserve muscle strength during weight loss, which matters for long-term metabolic health and physical function.

If you are new to exercise or have any health conditions that affect your ability to be active, speak to your healthcare provider before starting a new exercise programme.

Alcohol

There is no formal interaction between tirzepatide and alcohol in the Mounjaro prescribing information, and alcohol is not prohibited while taking tirzepatide [2]. However, there are a few things worth being aware of.

Nausea is one of the most common side effects of tirzepatide, particularly during the dose escalation period. Alcohol is generally known to worsen nausea and gastrointestinal symptoms, so drinking - especially in larger amounts - may make side effects harder to tolerate.

Both heavy alcohol use and tirzepatide are independently associated with an increased risk of pancreatitis - a serious inflammation of the pancreas. In January 2026, the MHRA strengthened warnings across all GLP-1 and GIP/GLP-1 medicines, including tirzepatide, following reports of severe, necrotising, and fatal pancreatitis. Heavy drinking should be avoided by everyone taking this medication [10].

Most people taking tirzepatide can drink in moderation without problems. If you have any concerns, speak to your doctor or pharmacist.

How much does tirzepatide cost?

If you are getting tirzepatide privately, the cost will depend on the dose you are prescribed. Each Mounjaro pre-filled pen contains four weekly doses (one month's supply). Prices vary by pack size - packs of one, two or three pens are typically available.

As a guide, prices at Pharmacy Online are as follows:

*Prices correct at time of writing but are subject to change.

The Bottom Line

What does this mean for weight loss?The evidence for tirzepatide is compelling. In the SURMOUNT-1 trial, participants on the highest dose lost up to 22.5% of their starting body weight over 72 weeks - compared to just 3.1% on a placebo. For many people, that difference is life-changing, not just in terms of the number on the scales, but in quality of life, physical function, and the potential to reduce or resolve weight-related conditions like high blood pressure and type 2 diabetes.

That said, tirzepatide is a prescription medicine, not a quick fix. It works best alongside sustainable lifestyle changes and under the guidance of a healthcare professional who can monitor your progress, manage any side effects, and adjust your dose over time.

Is tirzepatide right for you?

If you meet the eligibility criteria and are looking for a clinically proven treatment to support significant, sustained weight loss, tirzepatide is one of the most effective options currently available.

Start your online consultation for Mounjaro weight loss injections at Pharmacy Online today!

This article was written by Claudia Jackson RN Freelance Health Content Writer.

References

1. Jastreboff AM, Koenen M, Dent R et al. Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1). N Engl J Med. 2022;387(3):205-216. https://doi.org/10.1056/NEJMoa2206038

2. Eli Lilly and Company. Mounjaro (tirzepatide) Summary of Product Characteristics. Last updated April 2026. https://www.medicines.org.uk/emc/product/15481/smpc

3. National Institute for Health and Care Excellence. Tirzepatide for managing overweight and obesity (TA1026). 2023. https://www.nice.org.uk/guidance/ta1026

4. NHS England. Interim commissioning guidance: NICE TA1026 tirzepatide. https://www.england.nhs.uk/long-read/interim-commissioning-guidance-nice-ta1026-tirzepatide/

5. Frías JP, Davies MJ, Rosenstock J et al. Tirzepatide versus Semaglutide Once Weekly in Patients with Type 2 Diabetes (SURPASS-2). N Engl J Med. 2021;385:503-515. https://doi.org/10.1056/NEJMoa2107519

6. Jastreboff AM, Kushner RF, Blundell J et al. Tirzepatide versus Semaglutide for Obesity (SURMOUNT-5). N Engl J Med. 2025. https://doi.org/10.1056/NEJMoa2410184

7. Look M, Nissen S, Bhatt DL et al. Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study. Diabetes Obes Metab. 2025. https://doi.org/10.1111/dom.16275

8. Jastreboff AM, Aronne LJ, Ahmad NN et al. Tirzepatide for Sustained Weight Reduction in Adults with Obesity (SURMOUNT-4). JAMA. 2023;330(22):2180-2189. https://pmc.ncbi.nlm.nih.gov/articles/PMC10714284/

9. Horn DB, Linetzky B, Davies MJ, et al. Cardiometabolic Parameter Change by Weight Regain on Tirzepatide Withdrawal in Adults With Obesity: A Post Hoc Analysis of the SURMOUNT-4 Trial. JAMA Intern Med. 2026;186(2):157–167. https://jamanetwork.com/journals/jamainternalmedicine/fullarticle/2841273

10. MHRA Drug Safety Update. GLP-1 receptor agonists and dual GLP-1/GIP receptor agonists: strengthened warnings on acute pancreatitis, including necrotising and fatal cases. January 2026. https://www.gov.uk/drug-safety-update/glp-1-receptor-agonists-and-dual-glp-1-slash-gip-receptor-agonists-strengthened-warnings-on-acute-pancreatitis-including-necrotising-and-fatal-cases

11. NHS. How should I dispose of used needles or sharps? https://www.nhs.uk/common-health-questions/accidents-first-aid-and-treatments/how-should-i-dispose-of-used-needles-or-sharps/

12. Jastreboff AM et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity - A Phase 2 Trial. N Engl J Med. 2023;389:514-526. https://doi.org/10.1056/NEJMoa2301972

13. Eli Lilly and Company. Lilly's triple agonist, retatrutide, delivered powerful weight loss in pivotal Phase 3 obesity trial. Press release, 21 May 2026. Available at: https://investor.lilly.com/news-releases/news-release-details/lillys-triple-agonist-retatrutide-delivered-powerful-weight-loss 

14. Wharton S, et al. "Once-weekly semaglutide 7·2 mg in adults with obesity (STEP UP): a randomised, controlled, phase 3b trial." The Lancet Diabetes & Endocrinology. 2025. https://www.thelancet.com/journals/landia/article/PIIS2213-8587(25)00226-8/abstract