If you have just been prescribed a GLP-1 weight loss injection such as Mounjaro or Wegovy, the first three months can feel like a lot of unknowns. How quickly should the scales move? Is it normal to feel queasy? Why does the dose start so low? The early phase has its own rhythm, and knowing what is typical — and what is not — makes it far easier to navigate. This is an educational guide to what generally happens in the first 12 weeks, so you can have a better-informed conversation with a suitably qualified prescriber.
These are prescription-only medicines. Nothing here is a recommendation to start, source or dose them; it is simply an honest picture of the early journey to help you weigh up whether a properly supervised programme is right for you.
How weight loss injections actually work
GLP-1 medicines mimic a natural gut hormone, glucagon-like peptide-1, that your body releases when you eat. By copying and prolonging its effects, they work on several fronts at once. They act on appetite centres in the brain to reduce hunger and cravings, so you feel full sooner and eat less. They slow the rate at which the stomach empties, which sustains that feeling of fullness after a meal. And they help regulate blood sugar by prompting insulin release only when glucose is raised.
Mounjaro (tirzepatide) adds a second hormone pathway, GIP, alongside GLP-1 — a dual action that tends to produce greater appetite suppression than GLP-1 alone. Wegovy (semaglutide) works on the GLP-1 pathway on its own. In both cases, the effect is less about willpower and more about quietly turning down the biological drive to eat, which is why many people describe the change as their appetite simply becoming easier to manage.
The first principle: start low, go slow
The single most important thing to understand about the early weeks is titration. You do not begin on the dose that drives most of the weight loss. Instead, you start on a low introductory dose and step up gradually — typically every four weeks — until you reach an effective maintenance level that suits you. Starting low gives your digestive system time to adapt, which substantially reduces the nausea and other gut symptoms that a high starting dose would be far more likely to trigger.
That means the first 12 weeks are largely an adjustment period. You are building towards an effective dose, not sitting at one. Here is roughly how the early schedule looks for the two most common injections, though your prescriber may move faster or slower depending on how you tolerate each step:
| Weeks | Mounjaro (tirzepatide) | Wegovy (semaglutide) |
|---|---|---|
| Weeks 1–4 | 2.5 mg once weekly (starting dose) | 0.25 mg once weekly (starting dose) |
| Weeks 5–8 | 5 mg once weekly | 0.5 mg once weekly |
| Weeks 9–12 | 7.5 mg once weekly | 1.0 mg once weekly |
| After week 12 | Continues stepping up (up to 15 mg max) | Continues to 1.7 mg, then 2.4 mg maintenance |
The first 12 weeks are the on-ramp, not the destination. Both medicines keep climbing towards their full dose beyond the three-month mark, which is a big reason early weight change is usually modest.
Both injections are given once a week, subcutaneously — into the abdomen, thigh or upper arm — usually on the same day each week.
Week by week: what tends to happen
Weeks 1–4: settling in
The starting dose is intentionally gentle. Many people notice a subtle shift in appetite within the first week or two — smaller portions feel satisfying, and the constant background pull towards snacks softens. Some feel very little at first, which is normal too; the introductory dose is more about tolerance than dramatic effect. Mild queasiness, a slightly off appetite or occasional burping can appear in the days after your first injection and usually settle as the week goes on.
Weeks 5–8: the first step up
The first dose increase often brings a short return of digestive symptoms — a day or two of nausea or a change in bowel habit is common as your body meets the higher level. This tends to fade within a week or so. By now appetite suppression is usually more noticeable, and some people see the first meaningful movement on the scales, while others see very little yet. Both are within the normal range.
Weeks 9–12: finding your rhythm
By the end of the first 12 weeks most people have a clearer sense of how the medicine feels for them: how their appetite behaves, which foods sit well, and how each step up affects them. You are still below the maintenance dose at this stage, so it is worth resisting the temptation to judge the whole treatment on these early numbers. This is also when good habits around protein, hydration and movement start to pay off.
Side effects: what is normal and what is not
The most common side effects are digestive and, for most people, transient. Nausea is the one people report most often, alongside constipation, diarrhoea, occasional vomiting, reduced appetite (which is the intended effect) and some burping or wind linked to slower digestion. These are typically at their most noticeable during the first days on a new dose and then ease. Reassuringly, a large share of people experience either no nausea or only mild, manageable symptoms, and eating smaller, less fatty meals often helps.
A few less obvious effects can show up during rapid weight loss and are worth knowing about. Some people notice temporary hair shedding, and because a portion of any weight lost is muscle rather than fat, prioritising protein and doing some resistance exercise is widely recommended to help protect muscle mass. Facial volume loss — the so-called “Ozempic face” — is another consequence of losing fat quickly rather than a side effect of the drug itself.
Serious problems are uncommon but real, which is exactly why supervision matters. These medicines are not suitable in pregnancy or breastfeeding, or for people with a personal or family history of medullary thyroid cancer or the MEN2 syndrome. Rarely, they can be linked to inflammation of the pancreas (pancreatitis) or gallbladder problems, the latter partly a consequence of rapid weight loss. Severe or persistent vomiting, signs of dehydration, or severe, ongoing abdominal pain are not symptoms to push through — they warrant prompt medical advice.
Supporting your body through the ramp
The medicine does the heavy lifting on appetite, but the first 12 weeks go far more smoothly when your habits work with it rather than against it. A few widely recommended basics:
- Eat smaller, balanced meals with adequate protein, and stop when comfortably full rather than clearing the plate.
- Go easy on very rich, greasy or heavy food, especially in the days after a dose increase, as it can worsen nausea while digestion is slowed.
- Stay well hydrated, which helps with both nausea and constipation.
- Keep moving and add some resistance exercise to help preserve muscle while you lose fat.
- Keep a simple diary of your weekly dose, how you feel and any symptoms — it makes your prescriber reviews far more useful.
None of this replaces the medical relationship. GLP-1 injections are designed to be used alongside a reduced-calorie diet and more physical activity, under supervision, not as a standalone fix.
Red flags: when to contact your prescriber
Ongoing review is a feature of good care, not an inconvenience. Contact the prescriber supervising you if you have severe or persistent vomiting or diarrhoea, cannot keep fluids down, develop severe abdominal pain (particularly pain that spreads to your back), or notice anything that feels clearly out of the ordinary. Crucially, do not increase, decrease or stop your dose on your own — dose changes should be a shared decision with your prescriber, who can slow the schedule down if a step proves hard to tolerate.
A word on where these medicines come from
Because demand is high, a large grey market has grown up around weight loss injections, including counterfeit pens sold through social media and unregulated websites. Fake or improperly stored products carry genuine risks, and buying without a proper consultation removes the supervision that keeps the first 12 weeks safe. A legitimate route always involves an assessment by a suitably qualified prescriber, a genuine product, and follow-up — never a checkout button and no questions asked.
Making an informed choice
The first 12 weeks on a weight loss injection are best understood as a carefully paced beginning: a low starting dose, gradual increases, a few predictable bumps along the way, and results that build slowly rather than arriving overnight. Going in with realistic expectations — and a prescriber who is genuinely available to support you — makes all the difference. It is also worth reading our guide to what Mounjaro (tirzepatide) is and, before you commit, whether weight loss injections are safe and what tends to happen if and when you stop weight loss injections.
Above all, the quality of the service behind the prescription matters more than the brand on the pen. Use Clinic Finder to compare accredited clinics near you, check that your practitioner is a suitably qualified, properly regulated prescriber, and ask directly how they supervise the early weeks, manage side effects and support you if a dose does not suit you.
