Mounjaro Fifth Dose: Is it Safe?

Mounjaro is an injectable medication that is used to help with weight loss in adults who are obese. It has been licensed in the UK to be used as a weight loss treatment for those who have a body mass index (BMI) over 30 or have a BMI of 28 with a weight related health condition.

Mounjaro (more specifically its active ingredient, tirzepatide) belongs to a group of medications known as GLP-1 agonists. GLP-1 (glucagon-like peptide-1) is a hormone that helps control blood sugar levels and reduces your appetite by slowing down digestion. Mounjaro (tirzepatide) binds to similar receptors which allow it to manage your appetite.

When the correct Mounjaro dosage is combined with a healthy diet and physical exercise, Mounjaro can be an effective option for losing weight.

A Mounjaro pen comes with four pre-measured doses. However some users try to get an extra dose out of their pen. We are going to have a closer look into what this Mounjaro “fifth dose” is, and why it might not be good for you.

What is the Extra “Fifth Dose” of Mounjaro?

To better understand the ‘5th’ dose of Mounjaro, we should first have a look at the dosing schedule that is recommended for taking Mounjaro.

Mounjaro is available to purchase as either a single dose pen or as a Mounjaro KwikPen, which contains 4 fixed doses. Mounjaro is available in a range of differing strengths, from 2.5 mg to the maximum dose of 15 mg. 

The initial dose is 2.5 mg, which should be injected once a week for 4 weeks. Then, you can increase your dose of Mounjaro after 4 weeks have passed. Ideally, this will lead you up to your maintenance dose – the strength you will use going forward. 

For best results, this should be at the 15mg strength; however, some patients may find they experience stronger side effects at the higher doses, meaning some will choose to maintain at a lower dose. The decision of what dose of Mounjaro that you should be on is decided by your doctor. For further details on the dosing schedule for Mounjaro and how to take it, have a look at our dosing guide here. 

So, where does the 5th dose of Mounjaro come from? Each Mounjaro injectable pen is made up of 3ml of liquid and each dose of Mounjaro is 0.6 ml of liquid. This means that, in the Mounjaro KwikPen, there is enough liquid within the pen for 4 doses of Mounjaro. Patients may also see this “extra dose”, in other GLP weight injections such as Ozempic and Wegovy.

However, there is also a slight extra amount of liquid found in each pen. This is known as a buffer – its role is basically to ensure that there is enough liquid for 4 doses available when injecting Mounjaro. So, some users believe that there is an additional 5th dose within the Mounjaro pen due to the extra liquid that remains in the pen after taking the 4 doses of medication.

Eli Lilly, the manufacturer of Mounjaro, has announced that a new, modified version of the Mounjaro KwikPen will be released in April 2026. This pen will contain less of this extra liquid, so patients will no longer be able to inject a “fifth dose”.

Should I Use the Fifth Dose?

Weight loss injections have been carefully measured to ensure the correct amount of medication within each dose. However, in the extra buffer liquid, this might not be the case – due to the volume not being as measured, the concentration of Mounjaro that is contained within the buffer might be higher or lower than the required amount. This can lead to under or over-dosage, each of which carry their own risks. 

There is also an increased chance of damage to the Mounjaro pen by trying to get an extra dose from it. This means that it would no longer be usable, and would mean that another pen would need to be purchased.

Taking too much Mounjaro can increase your risks of having side effects. This is because your body is not used to the medication. So, starting at a lower dose then gradually increasing it will help give your body time to adjust to any side effects and allow you to experience the full benefits of Mounjaro. Some common side effects to look out for are:

• Nausea
• Diarrhea 
• Vomiting 
• Constipation 
• Indigestion

You can learn more about the safety of Mounjaro, and its side effects in the Patient Information Leaflet, or through our blog post on Mounjaro’s side effects.

By taking Mounjaro as per the recommended guidelines, this will be safer and will allow Mounjaro to be as effective as possible in helping you lose weight. The Mounjaro fifth dose is not suitable to continue your treatment.