What your Mounjaro starter kit actually contains — and what to do first

The moment the pen arrives: what you're actually holding

Picture this: your DPD parcel arrives in plain packaging. Inside is your Mounjaro KwikPen, a slim, pre-filled autoinjector made by Eli Lilly. The pen already contains tirzepatide solution for four weekly injections; you do not draw up a syringe or mix anything. What it does not contain are needles. Novo Nordisk and Lilly supply their pens without needles in the box, so you need compatible pen needles ordered separately before your first injection day.

The other item you'll need immediately is a sharps container. Used needles are clinical waste and cannot go into household recycling or a standard bin. A Mounjaro starter pack from some providers bundles these accessories together; whether yours does will be confirmed when your prescription is processed.

Once the pen is in your hands, put it in the fridge door, away from the freezer compartment, ideally not right against the back wall where temperatures fluctuate. That becomes the habit: fridge door, same shelf, every week.

The NHS tirzepatide medicines page has a clear overview of how the pen is used and what to expect in the first few weeks, and is a good companion read before your first dose.

What the 2.5 mg starting dose is actually for

The first pen's job is adjustment, not treatment at full effect. Tirzepatide works on two gut-hormone receptors (GIP and GLP-1) which slow gastric emptying and reduce appetite. Introducing that effect at lower intensity gives your digestive system time to settle before the dose climbs. Nausea and other gastrointestinal effects are most common in the first days after a new dose; starting at 2.5 mg reduces how sharply they hit.

After four weeks on 2.5 mg, a prescriber reviews how you've responded. If things have gone well, the dose typically increases to 5 mg. That clinical review matters, it is not automatic, and your prescriber may recommend staying at the current strength if your body is still adjusting. For a fuller picture of the treatment journey, the page on how tirzepatide works covers the mechanism and the wider evidence base.

One practical note on the dose-increase question: some patients feel frustrated that the starter dose produces only modest appetite change. That is expected. The therapeutic effect builds over the titration schedule; the 2.5 mg pen is part of a sequence, not a standalone course.

For context on what the full titration schedule can achieve, the SURMOUNT-1 trial (published in the New England Journal of Medicine) reported average body-weight reductions of around 20–21% at the highest dose over 72 weeks among adults without diabetes.

Side effects to watch for in the first few weeks

Most people using Mounjaro for the first time notice some gastrointestinal effects: nausea is the most common, followed by changes in bowel habit, burping, and sometimes fatigue or mild headaches. These tend to be most noticeable in the days immediately after an injection, then ease as the week progresses. By the time the second or third pen begins, many people find the pattern much more manageable.

A few effects warrant prompt attention. Severe, persistent stomach pain that spreads toward the back (especially alongside vomiting) should be assessed medically without delay, as pancreatitis is a known but infrequent risk with GLP-1 medicines. The MHRA highlighted this in its January 2026 Drug Safety Update and recommends patients be aware of the symptom. If something feels wrong, contact your prescriber or NHS 111, and any suspected side effect can be reported at Yellow Card.

Injection-site reactions are also possible, mild redness or tenderness where the pen was used. Rotating between your abdomen, thigh and upper arm helps. If you have questions about your response to the starter dose, our support team is available seven days a week and will connect you to a prescriber where needed.

Women using oral contraceptives should add a non-oral contraceptive method for the first four weeks of tirzepatide and for four weeks after each dose increase, because tirzepatide can reduce how well the pill is absorbed, a detail worth knowing about other unexpected effects of starting treatment too. The NHS England weight management injections guidance sets this out clearly alongside advice on HRT and surgery.

How a legitimate starter kit reaches you, and what to check

Mounjaro is a prescription-only medicine. The only lawful route to a starter kit in the UK is through a prescriber who has assessed your medical history, weight and suitability. Any seller offering tirzepatide pens without clinical assessment is operating outside the law, and the MHRA has warned repeatedly about counterfeit pens entering the market, some found to contain insulin rather than tirzepatide.

Before you order from any online pharmacy, verify it on the GPhC register at pharmacyregulation.org. A legitimate pharmacy will have a registered premises number displayed on its site. At nume, our GPhC registration number is 9012878, you can check it directly.

For an overview of what a three-month course involves, the Mounjaro three-month starter pack page explains the pens typically covered and what clinical review looks like over that period. And if you'd like to compare the cost of private treatment before deciding, our Mounjaro price comparison page covers what UK providers typically charge and what those figures actually include.

If you've already started tirzepatide elsewhere and are looking at next steps, the page on what to do once you've started tirzepatide covers transfer patients and ongoing clinical review. Ready to find out if Mounjaro is clinically suitable for you? Start your free consultation and a GPhC-registered prescriber will review your answers the same day.