Tirzepatide is a GLP-1 and GIP receptor agonist that’s used in weight loss drugs Mounjaro and Zepbound. It works by making you feel fuller and stopping you from overeating.
It’s suitable for people with a BMI over 30 or 27 with a weight-related health condition. It’s used until you reach your target weight and are able to maintain it. This may be a few months for some people, but a year or more for others.
Tirzepatide is a GLP-1 and GIP receptor agonist. It mimics the action of two natural ones found in your body, and works in three main ways to prevent you from overeating:
Tirzepatide also enhances insulin secretion, which helps improve blood sugar control. This helps you lose weight as it makes your body more efficient at burning sugar and less likely to store it as fat.
Tirzepatide starts working within hours and reaches peak concentrations between days one and two. After this, it stays at active levels in your body until around day seven – but this is when you’ll take another dose.
Although tirzepatide works fast, it can take a few weeks to achieve noticeable weight loss. Many people start seeing a significant difference by week four.
Tirzepatide is only available as an injection under the brand names Mounjaro or Zepbound in the USA.
Currently, there aren’t any oral forms of GLP-1 weight loss injections, and it’s unknown whether they’ll be available in the near future.
Who can take tirzepatide (Mounjaro)?
Those who have a body mass index (BMI) of 30, or a BMI of 27 with a weight-related health problem, like diabetes or cardiovascular disease, can take Mounjaro.
In rare cases, a clinician might prescribe it to someone with a BMI of 27 without any weight-related conditions, if they think it will be beneficial – but this would be considered off-label use.
Mounjaro was created by Eli Lilly, an American pharmaceutical company. It was approved for weight management in the UK in November 2023. Prior to this, Saxenda (liraglutide), another weight loss injection, was approved in 2017. GLP-1 weight loss injections have been around for a while and show many promising results.
While Saxenda was effective for weight loss, Mounjaro was a breakthrough as it only required weekly injections as opposed to daily with Saxenda. This made it more convenient for patients to use. Its dual actions also make it more effective than Saxenda or Wegovy , helping patients achieve weight loss in a shorter treatment period.
Preparing Mounjaro:
Injecting Mounjaro:
After injecting Mounjaro:
For detailed information on how to use Mounjaro, check the patient information leaflet.
You can inject Mounjaro any time of the day – just make sure it’s at a time where you’ll be free each week.
Some people may get side effects like nausea or an upset stomach within a couple of hours or the following day. Take this into consideration when scheduling your injection. For example, if you get side effects after 12 hours, consider injecting in the morning so you can sleep through them, or inject on the weekend so you don’t have to deal with side effects if you’re at work during the week.
If it’s been less than four days since the forgotten dose, just inject it as soon as you remember.
If it’s been more than four days since your last dose, skip the missed dose and take the next one at the scheduled time. You should have at least three days between two doses.
Taking a double dose or two doses close together can lead to overdose symptoms, such as very low blood sugar levels.
There are six doses of Mounjaro: 2.5mg, 5mg, 7.5mg, 10mg, 12.5mg, and 15mg.
You’ll typically increase your dose every four weeks until you reach the maintenance dose on week 21. There are some instances where you might remain on the same dose for a longer period.
Sometimes staying on the same dose for longer can be beneficial. For example, if the side effects persist, moving up a dose is likely to make them worse. So it’s best to remain on the lower dose until your body has had more time to get used to the drug. While this might slow down your weight loss progress, it means you’re more likely to stick with the medication in the long term.
Another reason to stay on a lower dose might be cost. The cost of Mounjaro increases with higher doses. At time of writing, the 2.5mg dose costs around £190 a month, compared to £280 for the 12.5mg and 15mg doses. It’s better to stay on a lower dose than not use Mounjaro at all, as stopping can cause you to gain back the lost weight.
Additionally, your clinician might suggest remaining on the 10mg maintenance dose, rather than the recommended 15mg maintenance dose. This is because studies show that there isn’t much difference in weight loss between the two doses (10mg and 15mg), yet it’s more cost-effective.
If you wish to stay on the same dose of Mounjaro for an extended period, message your clinician before your medicine is shipped, explaining why.
If you take a break from Mounjaro, you may not be able to resume at the same dose as before.
If it’s been more than four weeks, you’ll probably have to go back to the starting dose. This is because Mounjaro will have left your body, and its effect won’t remain. So when you restart, you’ll need to reintroduce gradually to reduce the risk of side effects.
If you plan to skip a week of Mounjaro, perhaps for a holiday, inform your clinician. This short pause shouldn’t significantly impact your weight loss, as the small amounts of medicine will remain in your body. But this should only be an occasional occurrence, not a regular habit.
To store an unused pen, keep it in the refrigerator between 2°C to 8°C until the expiration date.
Store used pens at room temperature (up to 30°C) for 30 days after the first use.
Always check the liquid before injecting to make sure it’s colourless or slightly yellow. If it looks cloudy or is frozen, don’t use the pen.
Dispose of the needle and the pen in a disposable sharps bin. This is typically a small yellow bin with a little lid on top. When the bin is full, dispose of it according to your local household waste regulations, usually by returning it to a pharmacy or designated collection point.
You should incorporate a healthy diet and exercise plan while using weight loss injections to get the best results. Aim to eat around 1500 calories a day if you’re a woman and 2000 if you’re a man, making sure a high proportion of it is protein (around 50g per day is a good guide). Include plenty of vegetables, fruits, nuts, and whole grains.
Try to exercise for at least 150 minutes per week, incorporating both moderate-intensity cardio and strength training to help maintain muscle mass.
You don’t have to completely cut out alcohol, but it’s good to limit your intake. Alcohol can worsen your side effects like nausea, vomiting, and fatigue, particularly during dose increases.
Additionally, it might hinder your weight loss by adding extra calories and potentially increasing your appetite. A typical beer or cocktail can contain around 200 calories. If you choose to drink alcohol, it’s important to include these calories in your daily intake.
Yes, there’s a chance you might get side effects when taking Mounjaro. Some of the more common side effects include nausea, vomiting, upset stomach, and digestive issues. These side effects should subside as your body adjusts to the treatment, but if they don’t, inform your clinician. After each dose increase, the side effects will temporarily worsen again, but they settle down within a few days.
Tirzepatide has no contraindications, but there are a couple of medications you should avoid when using Mounjaro.
You should be careful if you’re taking tirzepatide alongside your diabetes medication like insulin, acarbose, and glipizide – there’s a risk of getting hypoglycemia (low blood sugar levels).
Tirzepetide also decreases the effectiveness of oral contraceptives by delaying stomach emptying, which affects their absorption. It’s best to use a barrier method of contraception or switch to a non-oral form (like an IUS or IUD) for the first 4 weeks of treatment, and for 4 weeks after each dose increase.
Let your clinician know if you have any of the following conditions:
These conditions may make it unsuitable for you to use Mounjaro or may require closer monitoring.
Don’t use Mounajro if you’re pregnant, as it’s unknown whether the effect of tirzepatide will affect your baby. Likewise, it’s unknown whether tirzepatide passes into breastmilk, so you should avoid using it if you’re breastfeeding.
Make sure to use a barrier form of contraception or a non-oral contraceptive to prevent accidentally getting pregnant. If you happen to get pregnant, stop using Mounjaro and inform your doctor straight away.
You can get Mounjaro through the NHS or from a private pharmacy. The NHS has stricter criteria for Mounjaro, requiring a BMI of 40 or 37.5 with an accompanying risk factor, and they may have longer waiting times.
If you meet the criteria for Mounjaro and are looking to lose weight, you can get it through an online pharmacy such as Treated. Simply answer a few questions about your health and medical history, so a clinician can check whether Mounjaro is safe and suitable for you.
Tirzepatide. [Updated 2024 Feb 20]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-.
Tirzepatide Once Weekly for the Treatment of Obesity. New England Journal of Medicine, [online] 387(3).
Tirzepatide - Indications and Dose. [online] BNF.
MHRA authorises diabetes drug Mounjaro (tirzepatide) for weight management and weight loss.
Commissioning Statement: Liraglutide for obesity.
Evaluation and comparison of efficacy and safety of tirzepatide, liraglutide and SGLT2i in patients with type 2 diabetes mellitus: a network meta-analysis. BMC Endocrine Disorders, 24(1).
Tirzepatide - Conception and contraception.
Last updated on Aug 15, 2025.
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