Weight loss medications (Ozempic, Wegovy, Mounjaro): side effects, risks and drug interactions

New weight loss medications, such as semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro), were first developed for type 2 diabetes and have become best-sellers because they work so well. In clinical trials, many people lost 15% or more of their body weight. Yet these medications can cause side effects, some of them serious, and they can change how other medications and medical procedures work. This article explains the main risks of GLP-1 medications and what you can do to use them more safely.

How do GLP-1 weight loss medications work?

Semaglutide and liraglutide (Saxenda, Victoza) act like GLP-1, a natural gut hormone that increases insulin release after meals, reduces appetite and slows the emptying of the stomach. Tirzepatide acts on two hormone receptors, GLP-1 and GIP. Slower stomach emptying helps people feel full longer, but it also causes many of the side effects and interactions described below.

What are the most common side effects?

The most common side effects are nausea, vomiting, diarrhea, constipation, bloating and stomach pain. They are usually worst when starting the medication or increasing the dose, and they often improve over time. Severe vomiting or diarrhea can cause dehydration, which can harm the kidneys, so it is important to drink enough fluids and to tell your healthcare provider if you cannot keep fluids down.

To ease bowel symptoms, eat smaller meals, stop eating when you feel full, choose high-fibre foods, drink plenty of water and stay physically active. Seek medical help right away if you have severe or persistent stomach pain, vomiting, fever or no bowel movements, and tell the healthcare team that you are taking a weight loss medication.

Do weight loss medications increase the risk of pancreatitis or bowel obstruction?

Serious gut complications are uncommon, but they can occur. A Canadian study of health records compared people using semaglutide or liraglutide for weight loss with people using bupropion-naltrexone (Contrave), another weight loss medication. GLP-1 medications were linked with a higher risk of:

• pancreatitis (inflammation of the pancreas): about 9 times higher;
• bowel obstruction: about 4 times higher;
• gastroparesis (very slow stomach emptying): about 3.7 times higher.

These events were still rare. GLP-1 medications are also linked with gallbladder problems, such as gallstones, especially with rapid weight loss.

Other risk factors for pancreatitis include gallstones, heavy alcohol use, smoking and very high triglycerides. People who have had pancreatitis before should talk to their doctor about whether a GLP-1 medication is right for them. Sudden, severe stomach pain that spreads to the back, with or without vomiting, needs urgent medical care.

Do weight loss medications affect cancer risk?

The drug labels for semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro) say they should not be used by people with a personal or family history of medullary thyroid cancer or multiple endocrine neoplasia syndrome type 2 (MEN 2). This warning is based on thyroid tumours seen in rodents. It is not known whether these medications cause thyroid cancer in people. On the other hand, a large US study of people with type 2 diabetes found a lower risk of colorectal cancer with GLP-1 medications than with several other diabetes medications, such as insulin.

What should I know before surgery or a colonoscopy?

Because GLP-1 medications slow stomach emptying, food can remain in the stomach longer than expected. During anesthesia or deep sedation, this can increase the risk of stomach contents entering the lungs (aspiration).

A 2024 guidance from several US medical societies, including anesthesiology, gastroenterology and bariatric surgery, says that most people can keep taking their GLP-1 medication before a procedure. People at higher risk, such as those who recently started or increased the dose, or who have nausea, vomiting or bloating, may be asked to hold a dose or to follow a liquid diet for 24 hours before the procedure. Tell your surgeon and anesthesiologist about your weight loss medication well in advance and follow their instructions.

GLP-1 medications can also make it harder to clean out the bowel before a colonoscopy. In one US study, 19% of people taking a GLP-1 medication needed a repeat colonoscopy because of poor bowel preparation, compared with 11% of people not taking one. Ask your doctor whether you need a longer liquid diet or an adjusted bowel preparation.

Do weight loss medications affect birth control?

Yes, for some birth control methods. Tirzepatide (Mounjaro) slows stomach emptying and can lower the amount of hormone absorbed from birth control pills. Its label advises using a non-oral method, such as an IUD, implant, patch or ring, or adding a barrier method such as condoms, for 4 weeks after starting tirzepatide and for 4 weeks after each dose increase. Weight loss can also improve fertility, for example in women with polycystic ovary syndrome (PCOS), so unplanned pregnancies can happen. Because these medications should not be used during pregnancy, talk to your doctor if you are planning a pregnancy. Learn more about how hormones affect medications in women.

How do weight loss medications affect other medications?

Semaglutide and tirzepatide are not broken down by liver enzymes, so they have few direct drug interactions. However, they can affect other medications in several ways:

• Low blood sugar: taking a GLP-1 medication with insulin or a sulfonylurea (such as gliclazide or glyburide) increases the risk of low blood sugar. The dose of insulin or sulfonylurea may need to be lowered.
• Slower absorption of oral medications: slower stomach emptying can change how quickly other oral medications are absorbed. This matters most for medications that need tight dosing, such as warfarin.
• Levothyroxine: oral semaglutide (Rybelsus) can raise blood levels of levothyroxine (Synthroid), and the right levothyroxine dose can change with weight loss. Thyroid blood tests (TSH) should be checked.
• Medications that change with weight loss: as you lose weight, your blood pressure and blood sugar often improve, and the doses of medications for high blood pressure and diabetes may need to be lowered to avoid dizziness or low blood sugar.

Weight loss can also change how the liver processes some medications. In a small study of people who had bariatric surgery, the activity of several liver enzymes, including CYP3A, CYP2C9, CYP2B6 and CYP1A2, increased after weight loss. It is not yet known whether GLP-1 medications have the same effect, but it is a good reason to have your medications reviewed as your weight changes.

Do weight loss medications affect mood?

After reports of suicidal thoughts in people taking GLP-1 medications, regulators in Europe and the United States reviewed the evidence. In 2024, the European Medicines Agency concluded that the available evidence does not support a link between GLP-1 medications and suicidal thoughts or actions. Anyone who notices changes in mood, or thoughts of self-harm, should contact a healthcare provider right away. Learn more about Pillcheck for mental health.

Do weight loss medications affect the eyes?

In people with type 2 diabetes, a fast drop in blood sugar can temporarily worsen diabetic retinopathy, an eye disease. In a large trial, eye complications were more frequent with semaglutide than with placebo, mainly in people who already had retinopathy. People with diabetes should have regular eye exams, especially when starting a GLP-1 medication.

In 2025, the European Medicines Agency concluded that non-arteritic anterior ischemic optic neuropathy (NAION), a sudden loss of vision in one eye caused by reduced blood flow to the optic nerve, is a very rare side effect of semaglutide, affecting up to 1 in 10,000 people. Anyone who has a sudden loss of vision should contact their doctor right away.

What happens when you stop taking a weight loss medication?

Weight often comes back. In a trial of semaglutide for weight loss, people regained about two-thirds of the weight they had lost within a year of stopping the medication. Rapid weight loss can also include loss of muscle. A healthy diet, regular physical activity that includes strength training, and enough protein help protect muscle and keep weight off.

How can Pillcheck help?

Some companies offer genetic testing to predict response to GLP-1 drugs, but these tests are not yet clinically validated and do not predict the risk of serious complications, such as pancreatitis or vision loss. Many people who take GLP-1 medications also take other medications, such as statins, blood thinners, antidepressants and pain medications, that are affected by genes. A Pillcheck pharmacist reviews your genetic results together with all of your medications, checks for drug-gene and drug-drug interactions, and provides recommendations that you can share with your doctor as your health and weight change.

Pillcheck testing is performed in CAP- and COLA-accredited clinical genetic laboratories located in Canada. Pillcheck reports on 24 genes, increasing coverage with new guidelines and drug labels (see the genes tab), and covers 355 medications, which account for over 70% of dispensed prescriptions. The Pillcheck test panel ensures high accuracy for people of non-European and admixed ancestries. Your results last a lifetime, and report updates are free of charge.

Summary

• Ozempic, Wegovy, Mounjaro and other GLP-1 medications are effective for diabetes and weight loss, but they commonly cause stomach and bowel side effects.
• Pancreatitis, bowel obstruction and gallbladder problems are rare but serious.
• Tell your surgeon, anesthesiologist or gastroenterologist that you take a GLP-1 medication well before any procedure.
• Mounjaro can reduce the effectiveness of birth control pills: use a non-oral or barrier method when starting or increasing the dose.
• Doses of insulin, sulfonylureas, blood pressure medications and levothyroxine may need to change as your weight changes.
• People with diabetes should have regular eye exams, and anyone with sudden vision loss should seek care right away.
• Diet, physical activity and regular medication reviews help you get the best results.

Frequently asked questions

Should I stop Ozempic or Mounjaro before surgery?

Not always. Most people can continue, but your care team may ask you to hold a dose or follow a liquid diet before the procedure, especially if you recently started or increased the dose or have stomach symptoms. Always tell them that you take it and follow their instructions.

Can Mounjaro make birth control pills less effective?

Yes. Use a non-oral method or add a barrier method for 4 weeks after starting Mounjaro and after each dose increase.

Can I take Ozempic with insulin?

Yes, but the risk of low blood sugar is higher, and your insulin dose may need to be lowered. Check your blood sugar as directed and talk to your healthcare provider.

Do my genes affect how Ozempic works?

Some companies offer genetic tests to predict response to GLP-1 drugs, but these tests are not yet clinically validated and do not predict the risk of serious complications, such as pancreatitis or vision loss. Genes do affect many other medications that people often take with GLP-1 drugs, and a pharmacogenetic test can help make sure those medications are right for you. Learn more about genetic testing for GLP-1 drugs.

References

1. Sodhi M, Rezaeianzadeh R, Kezouh A, Etminan M. Risk of gastrointestinal adverse events associated with glucagon-like peptide-1 receptor agonists for weight loss. JAMA. 2023;330(18):1795-1797. doi:10.1001/jama.2023.19574
2. Kindel TL, Wang AY, Wadhwa A, et al. Multisociety clinical practice guidance for the safe use of glucagon-like peptide-1 receptor agonists in the perioperative period. Surg Obes Relat Dis. 2024;20(12):1183-1186. doi:10.1016/j.soard.2024.08.033
3. Yao R, Gala KS, Ghusn W, et al. Effect of glucagon-like peptide-1 receptor agonists on bowel preparation for colonoscopy. Am J Gastroenterol. 2024;119(6):1154-1157. doi:10.14309/ajg.0000000000002564
4. Wang L, Wang W, Kaelber DC, Xu R, Berger NA. GLP-1 receptor agonists and colorectal cancer risk in drug-naive patients with type 2 diabetes, with and without overweight/obesity. JAMA Oncol. 2024;10(2):256-258. doi:10.1001/jamaoncol.2023.5573
5. Ghasim H, Rouini M, Safari S, et al. Impact of obesity and bariatric surgery on metabolic enzymes and P-glycoprotein activity using the Geneva cocktail approach. J Pers Med. 2023;13(7):1042. doi:10.3390/jpm13071042
6. Marso SP, Bain SC, Consoli A, et al. Semaglutide and cardiovascular outcomes in patients with type 2 diabetes. N Engl J Med. 2016;375(19):1834-1844. doi:10.1056/NEJMoa1607141
7. Wilding JPH, Batterham RL, Davies M, et al. Weight regain and cardiometabolic effects after withdrawal of semaglutide: the STEP 1 trial extension. Diabetes Obes Metab. 2022;24(8):1553-1564. doi:10.1111/dom.14725
8. European Medicines Agency. Meeting highlights from the Pharmacovigilance Risk Assessment Committee (PRAC), 8-11 April 2024.
9. European Medicines Agency. PRAC concludes eye condition NAION is a very rare side effect of semaglutide medicines Ozempic, Rybelsus and Wegovy. June 6, 2025.