By Dr. Asnat Raziel, bariatric surgeon specializing in obesity treatment, metabolic health and healthy longevity; Director of the Multidisciplinary Center for Obesity Treatment, Assia Medical, Assuta Ramat HaChayal, Tel Aviv
GLP-1 side effects: what to know before you start
GLP-1 side effects are the topic my patients ask about most at their first appointment. The short answer is reassuring: most side effects are mild to moderate, show up mainly in the first few weeks, and fade with time. Still, there are a few rare side effects that are worth recognizing early. In this article I’ll explain what to expect, what the research shows, and when you should contact your doctor.
The focus here is on the GLP-1 family of medications, such as Wegovy and Mounjaro, because these are the drugs most patients receive today. For a broader overview of all the options, see my page on medical treatment for obesity.
How these medications work, and why they cause side effects
GLP-1 is a natural hormone released by the gut after a meal. The medications mimic it and act on both the brain and the digestive system. In the brain, they increase satiety and quiet what patients call food noise, those intrusive, persistent thoughts about food. In the stomach, they slow emptying, so food stays there longer.
That same mechanism explains most of the side effects. A stomach that empties slowly causes a feeling of fullness, nausea and sometimes vomiting. Changes in bowel movement can also lead to diarrhea or constipation. That is why treatment always starts at a low dose and is increased gradually.
Digestive GLP-1 side effects
Nausea, diarrhea, vomiting and constipation are the most common side effects across the entire drug family. They usually appear at the start of treatment or after a dose increase, and ease within a few days to a few weeks.
What the major trials showed
In the STEP 1 trial, 1,961 adults with obesity received either semaglutide 2.4 mg or placebo for 68 weeks. Nausea and diarrhea were the most common side effects, and most were mild and short-lived. Overall, 4.5% of those on the drug stopped it because of digestive side effects, compared with 0.8% in the placebo group.
The SURMOUNT-1 trial tested tirzepatide, the active ingredient in Mounjaro, in 2,539 participants. Here too, most side effects were digestive and appeared mainly during the dose-escalation phase. Depending on the dose, 4.3% to 7.1% stopped treatment because of side effects, compared with 2.6% on placebo. Put simply, out of every 100 patients, about 4 to 7 stopped treatment because of side effects.
How to ease nausea and constipation
- Eat small portions, slowly, and stop as soon as you feel full.
- Cut back on fatty, fried and very spicy foods.
- Drink water throughout the day, especially if you have diarrhea or vomiting.
- Add fiber, vegetables and daily physical activity to prevent constipation.
- Don’t increase your dose until the side effects of the previous dose have settled.
Less common GLP-1 side effects
Beyond the digestive system, there are a few rare side effects. They shouldn’t frighten you, but it helps to know about them so you can spot them in time.
Gallbladder and pancreas
Rapid weight loss raises the risk of gallstones, a topic I cover in detail in my article on gallstones and GLP-1 medications. Pancreatitis (inflammation of the pancreas) is rare, but it is listed among the warnings for these medications. That is why anyone who has had pancreatitis in the past needs a careful individual assessment, as I explained in my article on past pancreatitis and obesity medications.
Hair loss and muscle loss
Some patients notice hair shedding a few months after significant weight loss. It is usually temporary and linked to rapid weight loss and a diet low in protein. I discuss this further in my article on hair loss with GLP-1 medications. Some hair clinics offer scalp checks, and you can read more about The Hair Clinic on their website. However, hair loss during treatment should first be evaluated by your treating doctor.
In addition, part of the weight you lose is muscle mass. That’s why I advise my patients to eat protein at every meal and to do regular strength training throughout treatment.
Thyroid, pregnancy and other conditions
GLP-1 medications carry a warning about thyroid tumors, which were seen in studies in rats. According to the MedlinePlus information page on semaglutide, it is still unknown whether this risk also applies to humans. For this reason, these drugs are not given to people who have, or whose family members have had, medullary thyroid cancer or MEN2 syndrome. Treatment is also stopped before a planned pregnancy.
Another common question is whether these medications are linked to cancer. The data so far are largely reassuring, and I reviewed them in my article on satiety hormone drugs and cancer.
When to see a doctor right away
Most side effects are not dangerous. However, some signs call for urgent contact with a doctor or a visit to the emergency room:
- Severe, persistent abdominal pain, especially if it radiates to the back.
- Repeated vomiting that keeps you from drinking.
- Pain in the upper right abdomen, fever, or yellowing of the skin.
- A sharp drop in urine output or severe dizziness.
- A sudden change in vision or an allergic reaction.
How to minimize GLP-1 side effects
The most important rule is to go step by step. At our clinic, we increase the dose only when the patient feels well on the current one. Sometimes it makes sense to stay on a low dose for a long time, as long as it is meeting the goal.
Working with a dietitian also helps adapt your eating to a smaller appetite. Meanwhile, drinking enough, moving every day and sleeping well all help you feel better. These are all part of a healthy lifestyle, which strengthens the effect of the medication. If you are taking Mounjaro, I have gathered more tips in my article on Mounjaro and its side effects.
Finally, it’s important to know that stopping treatment abruptly usually leads to weight regain. In the STEP 1 extension, participants regained about two-thirds of the weight they had lost within a year of stopping the drug. So before stopping, it’s worth first trying a lower dose or treating the side effect itself.
The bottom line
GLP-1 side effects are common, but in most cases they are mild, temporary and manageable. Slow dose escalation, tailored nutrition and close medical follow-up reduce them considerably. Rare side effects such as pancreatitis or gallstones call for vigilance and prompt medical attention. If you are considering treatment, it’s best to do so with a doctor who knows these medications well.
The information in this article is general and does not replace personal medical advice.
Frequently asked questions
Do GLP-1 side effects go away over time?
In most cases, yes. Nausea and diarrhea appear mainly at the start of treatment and after dose increases, and ease later on.
Should I stop treatment because of nausea?
Not before talking to your doctor. Nausea can usually be eased by changing your diet, slowing the dose increases, or staying on a lower dose.
Do the new pills cause fewer side effects?
Not necessarily. In the ATTAIN-1 trial of oral orforglipron, 5.3% to 10.3% of participants stopped treatment because of side effects, compared with 2.7% on placebo.
Are there long-term GLP-1 side effects?
Semaglutide and tirzepatide have been in wide use for several years, and follow-up studies are still ongoing. That’s why I recommend regular medical follow-up throughout treatment. You can read more about my approach to treatment on the page about Dr. Asnat Raziel.
Sources
Once-Weekly Semaglutide in Adults with Overweight or Obesity
Tirzepatide Once Weekly for the Treatment of Obesity
Orforglipron, an Oral Small-Molecule GLP-1 Receptor Agonist for Obesity Treatment
