The Mounjaro Booklet: What the Patient Information Leaflet Actually Says

Where the PIL's information comes from

The patient information booklet that arrives with every Mounjaro pen is not marketing copy. It is a regulatory document, reviewed by the MHRA and required under the Human Medicines Regulations 2012 before any medicine can be sold in the UK. Its contents are derived directly from the Summary of Product Characteristics (SmPC), the full clinical dossier submitted to regulators. You can read both documents in full on the Mounjaro SmPC on the eMC, the electronic Medicines Compendium, which keeps them updated whenever Eli Lilly submits a revision.

The clinical evidence underpinning those documents is substantial. The SURMOUNT-1 trial (2,539 adults with obesity, 72 weeks, published in the New England Journal of Medicine) showed average body-weight reductions of around 20–21% at the highest 15 mg dose. Those figures fed directly into the risk-benefit assessment that earned tirzepatide its UK licence and its NICE recommendation (TA1026, published December 2024). Understanding that the booklet's side-effect frequencies and dosing guidance sit on that evidence base is the first step to reading it usefully.

For a broader overview of how tirzepatide works and what it is licensed for, our tirzepatide wiki covers the mechanism in plain language alongside the eligibility criteria.

Injection technique, storage and the details people skip

The sections most people overlook are often the most practically important. The PIL gives step-by-step instructions for each injection: choosing a site (abdomen, thigh or upper arm), rotating between sites to avoid tissue irritation, and the pre-injection checks for the pen itself. It also specifies exactly how to dispose of used pens, sharps containers are the correct method, never a household bin.

Storage is the section that catches people out. Mounjaro pens must be kept refrigerated at 2–8°C until use. The PIL specifies a limited window during which a pen may be kept at room temperature once removed from the fridge. That window differs between pen strengths and may be updated between PIL versions, so the booklet in your pack (not a secondhand figure from a forum or a previous prescription) is the figure to trust. A useful habit: keep the pen in the fridge door next to the milk, and only remove it shortly before your injection day.

If you are thinking about what eating patterns work alongside treatment, the Mounjaro cookbook guide and the Mounjaro recipe book page cover practical nutrition ideas that complement the advice in your PIL.

Side effects, warnings and when to get medical help

The PIL lists side effects by frequency, using categories defined by the European medicines regulator: very common (affecting more than 1 in 10 people), common, uncommon, rare and very rare. For tirzepatide, the very common and common categories are dominated by gastrointestinal effects, nausea, diarrhoea, constipation, vomiting, indigestion and burping. These tend to be most noticeable after starting treatment or after a dose increase, and for most people they ease within a week or two.

The PIL is also explicit about symptoms that need urgent medical attention. Severe, persistent abdominal pain that may spread to the back (with or without vomiting) can signal pancreatitis, a serious but infrequent reaction. In January 2026 the MHRA issued a Drug Safety Update reinforcing this warning for GLP-1 medicines including tirzepatide; the PIL reflects this guidance. Other urgent signals include signs of a serious allergic reaction and symptoms of gallbladder problems. The booklet tells you clearly: if in doubt, stop and seek medical help. You can also report any suspected side effect using the MHRA's Yellow Card scheme at the same link.

The PIL also lists contraindications and situations that require extra caution, a history of medullary thyroid carcinoma, MEN2 syndrome, severe gastrointestinal disease, or a personal history of pancreatitis. These are the conditions your prescriber assesses at consultation, before treatment begins.

What the booklet cannot do, and what a prescriber adds

The PIL gives you the standard framework. It cannot account for your specific medical history, the other medicines you take, or your starting weight and conditions. That is the prescriber's role. A prescriber will assess whether tirzepatide is appropriate, which starting dose applies, and how titration should progress in your case, guidance the booklet describes in principle but cannot personalise.

One area the PIL highlights but cannot resolve for you is contraceptive protection. Women taking oral contraceptives should discuss this with their prescriber before starting tirzepatide, because absorption of the pill may be reduced during the first four weeks of treatment and after each dose increase; a non-oral method is usually advised during those periods. The NHS England guidance on weight-management injections covers this and other practical considerations in detail.

If you want to understand the cost and access landscape around private treatment, the guide to Mounjaro costs in the UK sets out what affects pricing and what a legitimate prescription service should include. For a broader look at your options, including information about the Mounjaro booster, the weight-loss treatment overview covers all the licensed routes available through nume.

The booklet is your companion throughout treatment. Read it before your first injection, re-read the storage section each time a new pen arrives, and keep it to hand alongside our Mounjaro book resource, which brings together supporting guidance you may find useful between clinical reviews. If anything in it raises a question your prescriber has not yet addressed, that is a reasonable thing to bring to your next clinical review. Speak to our prescribers through a free consultation if you are not yet on treatment and want to understand whether tirzepatide could be right for you.