Mounjaro (Tirzepatide)

Tirzepatide acts on two gut hormone receptors, GIP and GLP-1. In practice this reduces appetite and slows stomach emptying, so people tend to eat less and feel full sooner.

It is given as a once-weekly injection using a pre-filled pen, starting at a low dose which is increased gradually to reduce side effects.

Licensed criteria are a BMI of 30 or above, or a BMI of 27 or above alongside a weight-related health condition such as type 2 diabetes, high blood pressure, obstructive sleep apnoea or raised cholesterol.

Meeting the criteria does not automatically mean it will be prescribed. That decision follows the clinical assessment.

1. Assessment

Twenty minutes covering your weight history, medical background, current medication and what you have already tried. Baseline bloods are arranged where indicated.

2. Decision

If tirzepatide is clinically appropriate, a private prescription is issued at the starting dose. If it is not, you will be told why, and what the alternatives are.

3. Titration

The dose increases gradually over months. This is deliberate — going up too quickly is the commonest reason people stop.

4. Review

Regular appointments to check tolerance, side effects and progress. Dose changes happen here.

5. Stopping

Discussed openly, including what typically happens to weight afterwards.

Reviews are part of the treatment, not an add-on

Tirzepatide is not a medicine to be issued and left alone. The dose changes over months, side effects cluster around those changes, and what happens to your weight is only half of what is being watched.

The schedule

• Around four weeks — tolerance, injection technique, whether nausea is manageable, and the first decision about increasing
• Every four to eight weeks during titration — each dose increase is a decision made at a review, not on request
• Every three months once the dose is stable, and longer-term thereafter

What is actually checked

• Weight and waist — waist matters as much as the scales, and sometimes more
• Blood pressure, which frequently falls, and existing blood pressure medication often needs reducing
• HbA1c if you have diabetes or prediabetes — and where you take a sulfonylurea or insulin, those doses usually need lowering to avoid hypoglycaemia
• Kidney function, particularly after any episode of vomiting or diarrhoea
• What you are actually eating. Appetite suppression makes it easy to eat far too little, and too little protein — which costs you muscle rather than fat
• Mood. Any new low mood or thoughts of self-harm should be reported promptly

Protecting muscle while you lose weight

A substantial share of weight lost on these medicines is lean tissue unless you act deliberately. That matters for how you feel now and for what happens when you stop.

• Protein at every meal — roughly 1.2 to 1.6g per kg of body weight daily for most adults, which takes planning on a suppressed appetite
• Resistance training twice a week. Not optional, and more important than cardio here
• Adequate fluid, since dehydration is the commonest avoidable complication

Come back sooner than your review if

• Severe or persistent abdominal pain, especially radiating to your back — possible pancreatitis, and this needs urgent assessment, not a message
• Vomiting you cannot keep fluids down against, which risks kidney injury
• Severe upper right abdominal pain, jaundice or pale stools — gallbladder
• New or worsening visual symptoms if you have diabetic retinopathy
• Signs of low blood sugar if you take insulin or a sulfonylurea

Stopping

Appetite returns, and weight regain is common and well documented. That is not a failure of willpower — it is what the evidence describes.

Plan the exit before you need it. Whether you taper, what habits are established, and what happens to eating patterns when the medicine stops are all better discussed at the start than on the day you run out.

Before anything else — what the NHS may already offer you

Check this first, because it may cost you nothing. Tirzepatide is available on the NHS for weight management through specialist services, on criteria that have been widening — and it is available for type 2 diabetes on separate criteria.

• Ask your NHS GP whether you meet current criteria and can be referred
• NHS tier 3 weight management services offer multidisciplinary support — dietitian, psychology, exercise — which private prescribing largely does not, and which meaningfully changes outcomes
• Bariatric surgery remains the most effective long-term treatment for severe obesity, and it is worth knowing that rather than assuming injections have replaced it

The other injectable

Semaglutide (Wegovy) — a GLP-1 agonist rather than the dual GIP/GLP-1 of tirzepatide. In head-to-head evidence tirzepatide produces greater average weight loss, but semaglutide has a longer track record and stronger published cardiovascular outcome data. Availability and price fluctuate for both.

Tablets

• Oral semaglutide — the same drug without needles, taken daily on a strict empty stomach, with somewhat less weight loss on average
• Orlistat — much older, far cheaper, and modest in effect. It blocks fat absorption rather than acting on appetite, and the side effects are what they sound like. Genuinely useful for some people, and available without prescription at a lower dose

What frequently matters more than the drug

• Reviewing what else you take. Several common medicines drive weight gain — some antidepressants, antipsychotics, older antihistamines, insulin, sulfonylureas, and steroids. Sometimes an alternative exists, and that conversation is free
• Treating an untreated cause. An underactive thyroid, sleep apnoea, PCOS, or a mood disorder all make weight loss substantially harder, and each is treatable in its own right
• Binge eating disorder, which is common, under-diagnosed, and responds to psychological treatment. An appetite suppressant in that setting can conceal a problem rather than address it

Where an injection is not the answer at all

If your BMI is below the licensed threshold, the honest answer is no — and any service that says otherwise is selling rather than assessing.

With a current or past eating disorder, this needs specialist input first, not a prescription.

What you pay us

£40 for the consultation. That is the assessment — and it is charged whether or not a prescription follows, because the assessment is the thing you are paying for.

Reviews are charged at the same £40, and they are not optional extras: dose increases are made at reviews.

What you pay the pharmacy

We do not dispense, we do not deliver, and we earn nothing from what you are prescribed. You take the prescription to a pharmacy of your choosing and pay them for the medicine.

Prices vary substantially between pharmacies, and rise with the dose. That last point catches people out: the cost at the starting dose is not the cost you will be paying six months later, and the higher maintenance doses are considerably more expensive.

• Ring two or three pharmacies and ask for the price at your dose before handing the prescription over. You are entitled to, and the differences are not small
• Ask what a full year at the maintenance dose would cost, which is the number that actually matters
• NHS prescription exemptions do not apply to private prescriptions

The cost nobody quotes

This is a long-term treatment, and stopping is generally followed by regain. So the honest budgeting question is not what a month costs but whether you can sustain it for a year or more — and what your plan is if you cannot.

Alongside the medicine there are reviews, occasional blood tests, and sometimes new clothes.

What we will tell you plainly

• If you may qualify on the NHS, we will say so, and referral through your own GP costs nothing
• If a cheaper treatment is reasonable for you, we will say that too. Orlistat is a fraction of the price and is right for some people
• If we do not think this is the right treatment, the fee still applies, because you paid for an assessment. We would rather say that in advance than have it be a surprise

Two warnings worth taking seriously

Never buy these medicines from a source that does not require a consultation. Counterfeit tirzepatide and semaglutide pens circulate in the UK, the MHRA has issued repeated warnings, and people have been hospitalised — several after injecting insulin sold as a weight-loss pen.

Be cautious of "packages" bundling months of medication into one upfront payment. They create a commercial reason to keep you on treatment and to move you up the doses, and they leave you out of pocket if the medicine does not suit you.

How long does Mounjaro take to work?

Appetite suppression usually begins within the first week — often after the very first injection. Meaningful weight loss takes months.

The two happen on different timescales, and confusing them is the commonest source of disappointment:

• Days 1–7: most people notice feeling full sooner and thinking about food less. Some notice nothing at 2.5mg, which is a starting dose designed for tolerance rather than effect
• Weeks 4–12: steady loss begins as the dose increases. Roughly 5% of body weight by week 12 is a typical trajectory
• Months 6–18: where the substantial results come from. Trial averages of around 20% were measured at 72 weeks, not at 12

If you have felt nothing at all by week four on 2.5mg, that is expected rather than a failure — the starting dose is a stepping stone. If you have felt nothing by week eight on 5mg, that is worth discussing.

How and where do I inject it?

Into the fat just under the skin — abdomen, thigh or the back of the upper arm — once a week, on the same day each week.

• Abdomen, avoiding a 5cm circle around the navel. The most commonly used site
• Front or outer thigh
• Back of the upper arm, which is easier if someone else is injecting

Rotate the site each week. Injecting into the same spot repeatedly causes lumpy, thickened tissue that absorbs the drug unpredictably — which then looks like the medicine losing effect.

It goes in at 90 degrees, no pinching needed for most people, and there is no need to inject with food or at any particular time of day. You will be shown the technique at your consultation, and the manufacturer's instructions come with the pen. Store it in the fridge; it can be out of the fridge for a limited period, and the leaflet specifies how long.

If you miss a dose: take it within four days, then carry on with your usual day. Past four days, skip it and take the next one as scheduled. Never double up.

Should I try to get a fifth dose out of the pen?

No. Please do not do this.

It is a widely shared idea online — that a residual amount is left in a KwikPen after four doses and can be extracted with a syringe. There are three reasons not to.

• You cannot measure it accurately. Drawing from a device not designed to be drawn from gives an unknown dose, which means either an ineffective week or an unintended increase with the side effects that brings
• Sterility is lost once the cartridge is breached, and you are injecting the result
• It is not what the pen was designed or licensed to deliver, and the manufacturer and MHRA both advise against it

If cost is the reason you are considering it, that is a conversation worth having openly. There may be an NHS route you qualify for, a different pharmacy price, or a treatment that suits your circumstances better. Improvising a dose is the wrong answer to a real problem.

When should the dose be increased, and can I stay on a low dose?

Increases are considered no sooner than every four weeks, and only at a review — and yes, staying on a lower dose is a legitimate choice if it is working for you.

The titration schedule exists to manage side effects, not to hurry you to the top. Standard practice is 2.5mg for four weeks, then 5mg, with further increases at four-week intervals as needed.

The maintenance dose is whatever dose is working with tolerable side effects. If you are losing weight steadily on 2.5mg or 5mg and feel well, there is no clinical reason to climb higher — higher doses mean more side effects, and the additional average benefit is not guaranteed for you personally.

Reasons to increase: weight loss has stalled for several weeks, appetite suppression has clearly faded, and side effects are manageable.

Reasons not to: you are still losing weight, or the last increase is still causing significant nausea. There is no prize for reaching the top dose.

What should I eat on it?

Less, but better — and protein is the part that matters most.

Appetite suppression makes it easy to eat very little without noticing, and what gets dropped first is usually protein. That costs you muscle rather than fat, which is the wrong kind of weight to lose.

• Protein at every meal — roughly 1.2 to 1.6g per kg of body weight daily for most adults. On a suppressed appetite this takes planning
• Smaller, more frequent meals. Large meals sit heavily because the stomach empties slowly
• Go easy on fat and fried food, which worsen nausea more than anything else
• Stop when you are full, not when the plate is empty. Overriding the signal is what causes vomiting
• Drink properly. Dehydration is the commonest avoidable complication, and reduced thirst comes with reduced appetite
• Fibre and fluid for constipation, which is as common as nausea and less talked about

Alcohol is worth reducing — it aggravates nausea, adds calories that bypass the appetite signal entirely, and can affect blood sugar.

Why does it cause diarrhoea?

Because it changes how quickly the gut moves, and the gut takes time to adjust.

Tirzepatide slows stomach emptying but also alters motility further down. The result is nausea and constipation for some people and diarrhoea for others — occasionally both, alternating.

It is usually worst in the days after a dose increase and settles within one to two weeks. Smaller lower-fat meals help, as does not increasing the dose again until things have settled.

When it is not just a side effect: if you cannot keep fluids down, if it lasts beyond a couple of weeks, or if you are becoming lightheaded or passing little urine, that risks kidney injury and needs assessment rather than patience.

What happens when I stop taking it?

Appetite comes back, and most people regain a substantial part of the weight. This is the best-documented and least-discussed fact about these medicines.

In the withdrawal studies, participants regained a large share of what they had lost within a year of stopping. That is not a failure of willpower — it is what happens when a drug suppressing appetite is removed and the underlying biology reasserts itself.

What that means practically:

• Treat it as a long-term treatment, in the way blood pressure medication is. Nobody expects to stop a blood pressure tablet and keep the benefit
• Build the habits while the medicine is making it easier — the resistance training, the protein, the eating patterns. Those are the parts that persist
• Plan the exit before you need it. Running out unexpectedly is the worst version of stopping

There is no withdrawal syndrome and no need to taper for safety. The question is entirely about what happens to your weight and appetite afterwards, and it is worth deciding in advance rather than discovering.

How much weight do people actually lose?

In the trials, an average of around 20% of body weight at the highest dose over 72 weeks, alongside diet and activity support. That is an average — some lose considerably more, some much less, and a minority lose very little.

Trial participants received structured dietary and behavioural support throughout. That is part of the result, not a footnote to it.

Is it available on the NHS?

Yes, on criteria that have been widening — and it is worth asking your NHS GP before paying privately. Availability differs by area and by service, and NHS routes generally include dietetic and behavioural support that private prescribing does not.

Will I have to stay on it forever?

Probably for as long as you want to keep the weight off, and that is the honest answer. Obesity behaves like a long-term condition: when the medicine stops, appetite returns and weight regain is common and well documented.

Nobody should start this expecting a short course that fixes things permanently. Plan for the long term, and plan for what stopping looks like.

Which is better, Mounjaro or Wegovy?

In head-to-head evidence tirzepatide produces greater average weight loss. But semaglutide has a longer track record and stronger published cardiovascular outcome data, and individual response and tolerance vary a great deal.

Price, availability and how you tolerate each matter more in practice than the average difference.

How bad are the side effects?

Nausea, vomiting, diarrhoea and constipation are common, worst in the days after a dose increase, and usually settle. They are the main reason people stop.

Slower titration helps considerably, and there is no prize for reaching the top dose quickly. Smaller meals, less fat, and stopping when full make a real difference.

What is the serious risk I should know about?

Pancreatitis. Severe abdominal pain, particularly radiating through to the back and with vomiting, needs urgent assessment the same day — not a message and not waiting for your review.

Gallbladder problems are also more common with rapid weight loss, and dehydration from vomiting can affect the kidneys.

Can I take it if I am trying to get pregnant?

No. It is not suitable in pregnancy, while breastfeeding, or when trying to conceive, and it should be stopped at least two months before conceiving.

If you use the contraceptive pill, this matters: absorption can be reduced around dose increases, so additional contraception is advised for four weeks after starting and after each increase. A non-oral method is often simpler.

Will I lose muscle?

Some, and more than people expect unless they act on it. A meaningful proportion of weight lost on these medicines is lean tissue.

Protein at every meal and resistance training twice a week are the countermeasures, and they matter more here than any cardio.

What if it stops working?

Weight loss plateaus are normal and not a sign of failure — the body adapts. It is worth reviewing dose, what you are eating, activity, sleep and other medicines before concluding the drug has stopped working.

Can I get it without a consultation?

No, and please do not try. These are prescription-only medicines requiring an assessment, and counterfeit pens circulate in the UK — the MHRA has warned repeatedly, and people have been hospitalised, some after injecting insulin sold as a weight-loss pen.

Any service selling these without a genuine assessment is one to walk away from.

Can I use it just to lose a stone before an event?

No. It is licensed for weight management in people meeting specific clinical criteria, and prescribing outside those is not something we will do.

If your BMI is below the threshold, the answer is no here — and a service that says yes is selling rather than assessing.