Mounjaro injections: How they work, the science, and dose titration

Mounjaro (Tirzepatide) is one of the most talked-about weight loss treatments available in the UK, and for good reason. It's licensed for weight management and works differently to older medications, which is why we're often asked exactly what it does in the body and what starting treatment actually looks like.

Here's a clear run-through of the science and the dosing schedule.

What makes Mounjaro different

Most GLP-1 medications act on a single hormone pathway. Mounjaro is a dual GIP and GLP-1 receptor agonist, meaning it copies two natural gut hormones instead of one: glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP).

Both are released after eating and play a role in appetite and blood sugar control. Activating both pathways at once is thought to be why Mounjaro tends to produce greater average weight loss than single-pathway medications in clinical trials.

The three main effects

• Slower stomach emptying. Food stays in the stomach for longer, so meals feel more filling and satisfaction lasts longer between them.

• Steadier blood sugar. Insulin is released more appropriately after eating, and the sugar spikes and crashes that often trigger cravings are smoothed out.

• Reduced hunger signals. Both hormone pathways act on the appetite centre in the brain, cutting cravings and reducing the urge to snack between meals.

  
  

What the trial evidence shows

Mounjaro's licensing was based on the SURMOUNT-1 trial, a 72-week study of adults with obesity who did not have diabetes.

At the highest dose (15mg), participants lost an average of 20.9% of their body weight, compared with 3.1% in the group given a placebo alongside diet and lifestyle support.

Lower doses in the same trial (5mg and 10mg) produced smaller but still substantial average reductions. As with any medication, individual results vary, and these figures are trial averages rather than a guarantee of any specific outcome.

• SURMOUNT-1 obesity trial results, ADA Scientific Sessions

• SURMOUNT-1 results, NEJM via Eli Lilly

Dose titration: what to expect

Mounjaro is started at a low dose and increased gradually. This isn't a delay tactic, it's how the digestive system is given time to adjust, and it's the main reason side effects during the first few weeks are usually much milder than people expect.

The typical schedule looks like this:

Weeks	Dose
1–4	2.5mg (starting dose, not yet at a treatment level)
5–8	5mg
9–12	7.5mg
13–16	10mg
17–20	12.5mg
21+	15mg (maximum dose, if needed)

Each step is usually held for at least four weeks. Not everyone needs to reach the maximum dose, some people reach their goals and stay comfortable on a lower maintenance dose, and your clinical team will adjust the pace if you're finding a step difficult to tolerate.

Common side effects during escalation

The most commonly reported side effects are gastrointestinal: nausea, diarrhoea, constipation and occasional vomiting. These tend to cluster around dose increases and usually settle within a week or two as the body adjusts. Taking injections with a lighter meal, eating more slowly, and avoiding very fatty or rich food around a dose increase all tend to help. If you experience side effects, speak with our Clinical Team who can advise you.

The bottom line

Mounjaro works by acting on two natural hormone systems at once, slowing digestion, steadying blood sugar, and quietening appetite, and the gradual dose increase is designed to make that process as manageable as possible. If you're weighing up whether it's the right option for you, our clinical team can talk through your medical history, expectations and the titration plan in detail.

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