What Happens in the First 3 Months on Weight Loss Injections

Step 1: Weeks 1–4 (the starter dose and what it is actually for

The first pen of Mounjaro contains 2.5 mg) and a question our prescribers hear most weeks is some version of: "Is this dose actually doing anything?" It is. The starting dose exists to let your digestive system adjust, reducing the chance of significant nausea before the therapeutic doses begin. Wegovy follows the same logic, opening at 0.25 mg. Neither of these is the dose that drives the bulk of weight loss; both are a carefully designed runway.

During this first month, appetite suppression is usually mild. Some people notice it clearly; others barely at all. Nausea, burping, or looser stools are common and, for most, manageable. Keeping meals smaller, staying well hydrated, and avoiding very fatty foods helps. Your weight may begin to fall simply because you are eating less without consciously trying, that reduction in appetite is the medicine working, even at a starter dose.

You can read more about how these medicines work and what to expect early on in our overview of weight loss injections. Storage matters from day one: pens must be kept refrigerated at 2–8°C, and the exact room-temperature window before use is set out in the Patient Information Leaflet supplied with your medication, always check it rather than guessing.

Step 2: Weeks 5–8, the first dose increase and growing momentum

Around week five, your prescriber will typically move you to the next rung: 5 mg for tirzepatide, 0.5 mg for semaglutide. This is where appetite suppression often becomes noticeably stronger, and where weight loss tends to pick up pace. It is also the point where side effects can briefly return, the body is adjusting to a new level, just as it did in week one.

Protein intake and keeping active matter here. When appetite falls sharply, it is easy to under-eat in ways that cost muscle rather than fat. NHS guidance on healthy weight emphasises that what you eat alongside treatment affects the quality of weight lost, not just the quantity. The treatment overview on our site covers the lifestyle factors that sit alongside injectable medicines.

By the end of week eight, many people are already recording measurable weight loss. The SURMOUNT-1 trial, which studied tirzepatide over 72 weeks, reported average body-weight reductions that continued accumulating well past this point, three months is the beginning of the curve, not the peak. You can see the full SURMOUNT-1 data in the New England Journal of Medicine.

Step 3: Weeks 9–12, a clearer picture and the question of continuing

By the end of three months, you and your prescriber have real data to work with. How much weight have you lost? How well have you tolerated the current dose? Are there reasons to push to the next strength, hold steady, or review the approach entirely?

NICE's appraisal of tirzepatide (TA1026) notes that if less than 5% body weight has been lost after six months at the highest tolerated dose, continuing should be reviewed, a reminder that these medicines require ongoing clinical oversight, not just an initial prescription. That review process is built into how a regulated service works: every repeat involves a prescriber looking at your progress, not just reissuing automatically.

If you are considering how a three-month supply is structured practically, our page on a three-month supply of weight loss injections explains what that typically involves. It is also worth understanding early on that treatment is not indefinitely open-ended for everyone, the conversation about duration starts at the three-month mark, not the end of treatment.

What three months does and does not tell you

Three months is a meaningful window, but it is not the full story. The largest clinical benefits in trials accumulated over 68–72 weeks. People who saw modest early results sometimes went on to achieve much greater reductions as dose escalation continued. Conversely, some people find side effects limit their dose, which affects what is achievable.

Weight loss is not linear. Weeks where the number on the scale barely moves are normal and do not mean the medicine has stopped working, fat loss and water shifts do not always register together. Keeping a log of how your clothes fit, how your energy levels have changed, and what your appetite feels like gives a fuller picture than the scales alone.

These medicines are prescription-only for good reason: the prescriber's role is not just gatekeeper at the start but an ongoing clinical relationship across the months. Buying from an unverified seller (one that supplies medicines without a prescription or clinical assessment) removes that entirely. The MHRA has warned repeatedly about counterfeit and improperly supplied GLP-1 pens, and in 2025 law enforcement seized approximately 20 million doses of illegally traded weight loss medicines worth around £45 million. The right question at three months is whether your treatment is working safely under proper supervision, not whether you found a cheaper source. You can verify any online pharmacy's registration on the GPhC register.

If you are weighing up how three months compares to a shorter trial period, our page on what one month on a weight loss injection involves covers the very early phase in more detail, and if you are curious whether taking weight loss injections for just a month is a viable approach, that question is addressed there too. For those interested in a less frequent dosing schedule, we also have a dedicated page on how a once-a-month weight loss injection works. And for a broader look at all licensed options, the weight loss injections guide is a good place to start before consulting a prescriber.