Weight loss injections side effects: What to expect

Weight loss injections can be a highly effective tool for managing excess weight, but they do come with potential side effects.

Although this might sound worrying at first, don't let it deter you just yet.

Most of these side effects are mild and temporary. And understanding what to expect can help you prepare for your journey and get the most out of your treatment with minimal discomfort.

Read on as we go through the side effects you might experience while on a weight loss medication, how long they usually last, and tips for managing them safely.

How do weight loss injections work?

There are a few different types of weight loss injections, each working slightly differently in terms of active ingredients, dosage, and effectiveness.

Generally speaking, these medications work by mimicking hormones in your body that regulate hunger and blood sugar. When you take them, your digestion slows down and, as a result, you feel fuller sooner and for longer. Your insulin response also improves, which reduces energy crashes that can trigger snacking or overeating.

The result? You're less likely to pick at food between meals or give in to cravings that aren't driven by genuine hunger—like eating when you're bored, stressed, or sad—making it easier to stay in a calorie deficit.

Types of weight loss injections

There are several prescription medications available in the UK to help manage excess weight:

Wegovy

Wegovy is a once-weekly injection containing the active ingredient semaglutide, which mimics the naturally occurring GLP-1 hormone to stabilise blood sugar levels, stimulate satiety, and encourage weight loss.

Wegovy is highly effective—and in clinical trials, patients on the highest dose lost up to 20.7% of their initial body weight [1].

Mounjaro

Mounjaro contains the active ingredient tirzepatide, which targets 2 digestive hormones, GLP-1 and GIP, to regulate glucose and register feelings of fullness in the brain.

Along with Wegovy, Mounjaro is considered a gold standard treatment, with studies showing up to 20.9% weight loss in patients on the highest dose [2].

Ozempic

Ozempic is a weekly injection containing semaglutide, originally developed to treat diabetes by helping to regulate blood sugar levels.

Although not licensed in the UK for weight loss, some patients have used it off-label for that purpose. Keep in mind that availability tends to be limited, since diabetic patients get priority.

Saxenda

Saxenda is a daily injection with the active ingredient liraglutide, another GLP-1 receptor agonist.

The main difference is that its effectiveness isn't as potent or long-acting as the newer alternatives. In one study, participants prescribed liraglutide in combination with lifestyle changes lost 10% of their initial weight [3].

Do weight loss injections have side effects?

Yes, similar to any other medicine, weight loss injections come with side effects.

Most are mild and typically not a reason for concern. They're a result of your body adjusting to the medication, and over time, should subside.

There are more serious side effects that, although rare, are still worth knowing. We'll dive into these in just a bit, but first, let's go through the ones you likely don't have to worry about.

The most common side effects

Most side effects of weight loss drugs are digestion-related, including:

Nausea and vomiting

Just over 1 in 5 people experience nausea as a side effect of Mounjaro, and 44% of people in clinical trials felt nauseous while on Wegovy [6][7].

Nausea is one of the most prevalent side effects of weight loss drugs, and it can be triggered by the signals they send your brain to tell you you're full, which can unintentionally affect the part that controls nausea and vomiting (aka the area postrema) [8].

Plus, since these injections slow down gastric emptying and make you feel fuller for longer, they can bring on a sense of heaving or queasiness.

Constipation, bloating, and gas

Since these medications slow down the rate at which your stomach empties, they cause food to stay in your digestive system for longer. The result? Constipation, bloating, and, for some people, a build-up of gas.

And it doesn't help that, with a reduced appetite, you may not get enough fibre, which would otherwise support healthy stool production.

Stomach pain or cramping

Once again, the delayed gastric emptying is mainly to blame here. That, along with changes in gut muscle movement [9] and the constipation and gas we mentioned before, can all lead to abdominal cramps or a feeling of “tightness".

While mild discomfort isn't usually a reason for concern, severe stomach pain can indicate something more serious is going on.

If you notice serious digestive problems while on a weight loss drug, it's best to err on the side of caution and seek medical attention ASAP.

Indigestion or reflux

Since food stays in your stomach for longer, there's an increased risk for acid build-up, gas production, and reflux. This is because retained food creates more pressure in your stomach, which in turn can force acid to backflow into the oesophagus.

This side effect can be exacerbated by certain foods—like greasy or spicy dishes—so it helps to pay extra attention to what you eat.

Other non-gastrointestinal side effects include:

• Headaches
• Fatigue
• Dizziness
• Injection site reactions
• Mood changes
• Body aches
• Dry mouth

Some people also report hair loss as a side effect of these injections, although there's no evidence linking the two. Most likely, it is the rapid weight loss that can trigger a condition called telogen effluvium, which causes excessive hair shedding.

Rare but possible side effects

Although uncommon, more serious side effects can occur and should be taken seriously. These include:

• Severe abdominal pain
• Increased heart rate
• Change in vision in people with type 2 diabetes
• Kidney problems or kidney failure
• Gallbladder disease, including gallstones
• Hypoglycemia (low blood sugar)
• Pancreatitis
• Unusual or allergic reactions, with symptoms such as skin rash, itchiness, and flushing, or in severe cases, trouble breathing/swallowing

There's also a potential link between weight loss injections and thyroid cancer, although this has only been seen in animal studies, not humans [4][5].

Still, tirzeparide and semaglutide injections shouldn't be prescribed to patients with a personal or family history of medullary thyroid carcinoma (MTC).

How long do weight loss injection side effects last?

There isn't one set answer to this question, and everyone's experience with weight loss injections is different.

That said, milder side effects usually subside within a few days or weeks, as your body gets used to the medication.

In the meantime, there are a few things you can do to minimise the discomfort:

• Start at a lower dose and increase gradually, as your doctor prescribes
• Eat smaller, more frequent meals
• Avoid greasy, spicy, or very rich foods
• Limit how much you drink alcohol
• Stay hydrated and focus on fibre for digestive health

More severe side effects can last longer, and it's important you manage them with guidance from your doctor.

What happens after you stop weight loss injections?

If you decide to stop taking weight loss drugs, whether that's because you've reached a healthy weight or want to give your body a break from the side effects, the main thing you'll notice is an increase in appetite.

There will also be some digestive adjustments, and any lingering side effects like nausea or bloating should resolve.

Blood sugar levels can increase or be harder to keep stable, too, especially in people with type 2 diabetes. If you were living with high blood pressure before starting the injections, it might spike again.

Will you gain the weight back?

The million-dollar question! The answer is no, not necessarily. Not if you have a strategy to transition off your medication, rather than stopping cold turkey.

We always recommend reducing your dosage with a doctor's guidance to reduce the risk of rebound symptoms, including weight regain.

It's also important to use weight loss injections as a piece of the puzzle, and not the only solution. The reality is that your results will always be more effective and sustainable if you pair your medication with a healthy, high-protein diet and regular strength training.

You'll be able to build muscle mass while losing fat. The more muscle you have, the faster and more efficient your metabolism will be, even after you stop taking the injections.

Balancing results and safety

Achieving lasting weight loss results safely means combining the right medication with lifestyle support. That's why, at Juniper, we always put your safety and well-being first.

With the Juniper Programme, you can access weight loss drugs tailored to your health and goals, whether that's Wegovy or Mounjaro.

Our app helps you track progress, adjust medication as needed, and access expert support. You'll also get structured tools to not only lose weight, but build healthier habits you can take with you for life—like healthy eating and regular exercise.

And, of course, our team of clinicians and health coaches will be there every step of the way to answer all your questions, address any concerns, and help you manage any side effects you may experience.

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