GLP-1 side effects: a clear overview

The most common side effects of GLP-1

Gastrointestinal complaints are by far the number one. According to the Farmacotherapeutisch Kompas (the Dutch national drug formulary) of Zorginstituut Nederland (the Dutch National Health Care Institute), 30% to 50% of users experience nausea at the start of treatment. That sounds like a lot, but there is a reassuring side to it: the complaints are part of the adjustment phase and decrease within a few weeks for most people.

The pattern is predictable: complaints peak at the start and after every dose increase. If you know that, it's less of a shock and you can steer on it together with the doctor, for example by postponing a dose step. You'll read below exactly how these complaints arise.

What does GLP-1 do to your stomach and gut?

GLP-1 slows your gastric emptying and suppresses your feeling of hunger. As a result you naturally eat less, but this same action also causes most of the side effects.

GLP-1 mimics a gut hormone that your body makes itself. Your stomach passes food on to your intestines more slowly, so you feel full sooner and for longer. If you still eat your old portions, your body responds with nausea or a bloated feeling. Your gut also has to get used to the new pace: that's why some users suffer from diarrhoea at the start, and others from constipation.

So these gastrointestinal complaints are not a sign that something is going wrong, but that the medication is working. After the adjustment phase, the stomach and gut usually find a new balance. The experience does differ per active substance, though.

Side effects per medication: semaglutide, liraglutide and tirzepatide

All GLP-1 medications share the same basic profile of gastrointestinal complaints. The differences lie mainly in the administration, the intensity and a few medication-specific points of attention.

Semaglutide (Ozempic, Wegovy and Rybelsus) is the best-known active substance. The side effects of semaglutide are mainly nausea, diarrhoea, constipation and fatigue during the build-up phase. Since June 2025, the very rare eye condition NAION has also officially been included in the product information, reports the CBG (the Medicines Evaluation Board, the Dutch medicines regulator): it affects about 1 in 10,000 users. We supervise semaglutide treatments through medical weight loss with injections.

Liraglutide (Saxenda) is injected daily instead of weekly. The side effects of Saxenda are very similar to those of semaglutide; because of the daily injection, reactions at the injection site, such as redness or itching, are more common. You can read more about this treatment under medical weight loss with injections.

Tirzepatide (Mounjaro) acts on two hormones at once: GLP-1 and GIP. The side effects of tirzepatide are comparable to those of the other medications: mainly gastrointestinal complaints, usually mild to moderate and concentrated around the build-up phase. You can read how we work with it under medical weight loss with injections.

The doctor decides which medication is suitable for you based on your health and your situation. Fortunately, what you can do yourself about complaints applies to all three.

How to reduce the complaints

You can prevent or ease most side effects with a calm dosing schedule and adjusted eating habits. This works best in practice:

• Build up the dose slowly. The build-up phase exists for a reason: your body gets used to it step by step.
• Eat small portions and stop at the first sign of feeling full.
• Avoid fatty, fried and very sweet food; it now stays in your stomach for extra long.
• Drink enough water, especially with diarrhoea or constipation.
• Keep moving every day; that keeps your bowels going.

At Qlinic, our regular BIG-registered doctor (the BIG register is the Dutch register of healthcare professionals) draws up your dosing schedule and adjusts it based on your complaints. Every 4 weeks you speak to the doctor personally in a follow-up, and you can get in touch about side effects in between. That way you never have to tinker with your dose yourself.

Not sure whether medical weight loss suits you? Our BIG-registered doctor assesses your situation personally and also supports you if you experience side effects. Take the eligibility check →

Preventing muscle loss while losing weight

With rapid weight loss you lose not only fat, but also muscle mass. That applies to every strict diet and therefore also to losing weight with GLP-1.

The mechanism is simple: your appetite decreases sharply, so without noticing you take in too little protein. If you lose a lot of muscle mass, your metabolism drops and your result becomes more vulnerable. The rapid loss of volume in your face, which has become known as Ozempic face, is also linked to this rapid weight loss. Read more about Ozempic and your eyes.

Prevention is very doable: eat a full portion of protein at every meal, such as quark, eggs, fish, chicken or pulses, and train your muscles 2 to 3 times a week. This is especially important for women going through menopause, because muscle mass already decreases faster in that phase. During the follow-ups, the doctor checks with you whether your weight loss is progressing healthily and whether you're getting enough nutrition.

Serious side effects: when should you call a doctor?

Serious side effects of GLP-1 are rare, but you should know the signals. Contact your doctor straight away if you have:

• Severe, persistent abdominal pain, especially if it radiates to your back (possibly pancreatitis)
• Pain in the upper right of your abdomen, possibly with fever (possibly gallstones)
• Persistent vomiting that makes you dehydrated
• Suddenly reduced or blurred vision

Bijwerkingencentrum Lareb (the Netherlands Pharmacovigilance Centre) warns that rare but serious side effects such as gallstones can occur, and stresses that you should only use these medications under the supervision of a doctor. And the stories about low mood? In 2024, the European medicines authority EMA examined more than 150 reports and found no evidence of a link between GLP-1 and suicidal thoughts.

Admittedly, this list can be off-putting. Bear in mind that these are rare situations and that quick access to your doctor makes the difference. At Qlinic you can go to your own doctor with signals like these, and pausing or stopping is always possible in consultation.

How long can you use GLP-1?

You can use GLP-1 long-term, as long as the medication works well and you tolerate it well. Your doctor assesses this periodically: after the build-up phase, the doctor looks at whether the treatment is effective enough and whether continuing makes sense.

If you stop, your feeling of hunger returns and your weight can go up again, because the underlying causes haven't gone away. In our article on why you're not losing weight you can read which factors play a role. That's why a good treatment is never about the medication alone, but also about diet, exercise and behaviour. At Qlinic, you also always decide yourself, in consultation with the doctor, whether to pause or stop.

Want to lose weight with a doctor who is also there when you have complaints? At Qlinic you have one regular BIG-registered doctor who speaks to you personally every 4 weeks and whom you can contact about side effects. Take the eligibility check →

Frequently asked questions about GLP-1 side effects

What are the downsides of GLP-1?

The main downsides are gastrointestinal complaints in the initial phase, the costs of a treatment and the risk that the weight returns if you stop without adjusting your lifestyle. In addition, you inject the medication weekly or daily, and medical supervision is always necessary.

How long can you use GLP-1?

As long as the medication is effective and you tolerate it well, you can keep using GLP-1 long-term. Your doctor evaluates the treatment periodically and adjusts the plan if the effect is disappointing or the side effects are too heavy.

Can I lose weight with GLP-1?

Yes, GLP-1 medication supports weight loss by suppressing your feeling of hunger and slowing your gastric emptying, in combination with healthy food and exercise. A doctor assesses beforehand whether it is medically suitable for you.

What does GLP-1 do to your gut?

GLP-1 slows your gastric emptying and changes the pace of your digestion. As a result, in the first weeks you may get diarrhoea or, on the contrary, constipation. These bowel complaints are almost always temporary and decrease as soon as your body has adjusted.