What to Expect When You Start Taking Weight Loss Medication

04 August, 2025  |  Jane Thomas - Marketing

Thinking about trying some kind of weight loss treatment for the first time? The thought of starting any new medication can be a little daunting, particularly one that can have a significant impact on the way your body craves (and processes) food - but we're here to guide you through the process.

In this article, we're going to take a close look at what to expect when starting weight loss drugs, be it a pen like Wegovy, Mounjaro or Saxenda, or a tablet like Xenical (Orlistat). Below, we'll not only cover the effects these medications can have on your body - and how soon you're likely to see changes - but run through the potential side effects and how you can manage them effectively.

What happens when you start weight loss medication?

There's no single straightforward way to answer this question, because it depends on which weight loss treatment you choose.

• Weight loss injections - like Mounjaro and Wegovy - work by limiting your appetite. After eating a meal, you'll feel fuller for longer when using these medications, and you should feel less inclined to snack in between meals.

• As well as limiting your appetite, Mounjaro also boosts your body's insulin sensitivity, allowing your blood sugar levels to gradually decrease. This means your body absorbs less fat through digestion.

• In contrast to weight loss injections, weight loss tablets like Xenical (Orlistat) act purely on your body's digestive system. By inhibiting the enzyme lipase in your stomach and pancreas, approximately one third of the fat you consume through food is passed through your gut instead of entering your bloodstream.

As you can tell, all of the weight loss pens and tablets available in the UK today work slightly differently - so you'll have a different experience depending on which medication you choose.

For instance, if you take Wegovy, you probably won't notice any significant physical changes in your body apart from gradual weight loss (provided you stick to a balanced diet and a healthy lifestyle, of course). The main change you'll notice will be to your appetite.

Because Mounjaro and Xenical (Ortlistat) work on your digestive system, you're more likely to notice physical changes besides weight loss - things like oily stools, more regular bowel movements, bloating and low blood sugar levels (which can cause a variety of symptoms).

How quickly will I begin to lose weight?

Again, it depends on a variety of factors:

• The medication you choose and the dosage you're placed on

• Your diet

• How much exercise you get

• How much of your weight is 'water weight' from stored carbohydrates

To give you a rough idea, one study found that Mounjaro users lost approximately 1% of their body weight on average each week, ultimately losing somewhere between 15-20% of their starting weight over a 72-week period.

What are the common side effects of weight loss injections?

Wegovy

The most common side effects of Wegovy include nausea, diarrhoea, vomiting, constipation, abdominal pain, headache, fatigue, upset stomach, dizziness, bloating, belching, gas, stomach flu, heartburn, and a runny nose or sore throat. 

Find out more about the potential side effects of Wegovy and how to use it.

Mounjaro

Mounjaro's most common side effects include nausea, diarrhoea, decreased appetite, vomiting, constipation, indigestion and stomach pain.

Discover more about Mounjaro and how it works, as well as the potential side effects.

How to manage the side effects of Mounjaro and Wegovy

Users of Mounjaro and Wegovy most commonly experience side effects when they first start using the medication, and they often gradually dissipate over time.

To minimise the impact of any side effects, make sure you…

• Start on a low dose and gradually work your way up to a higher dose if required (and under the guidance of your doctor or pharmacist)

• Eat a balanced, healthy diet that's low in fat

• Stay hydrated and drink plenty of water - this can help to minimise any side effects

• Find the best possible time to inject the medication when the side effects are less likely to bother you. Many people choose to use Mounjaro or Wegovy in the evening, allowing them to get some sleep and wake up feeling better the next morning

What side effects are commonly associated with weight loss tablets?

Xenical (Orlistat) tablets can cause oily stools, frequent bowel movements and vitamin deficiencies (particularly vitamins A, D, E and K). Taking multivitamin supplements and switching to a healthy, low-fat diet can help minimise these side effects.

Find out more about the side effects and efficacy of Xenical (Orlistat).

Want to find out more about weight loss medications and establish which one is right for you?

If you're not sure where to begin, why not explore our full range of weight loss treatments and begin your free online consultation with one of our pharmacists?

With iQ Doctor, there's no need to book a face-to-face appointment; simply select a treatment that you feel meets your needs, hit 'Start Consultation' and we'll guide you through the rest. One of our experienced pharmacists will be on-hand to help you get the medication (and dosage) that's right for you - and when you're ready to get started, we'll deliver your chosen treatment to your door the very next day in discreet packaging.

Put simply, there's no easier way to get your weight loss journey started. If you have any questions, don't hesitate to get in touch - we're here to help!