Millions of people worldwide – as many as 1 in 8 adults in the U.S. – are on GLP-1 medication, such as Ozempic, Mounjaro, Wegovy and Zepbound, for treating diabetes or for weight loss.

These drugs are adding a medical twist to religious observances that involve fasting, such as the Jewish holiday of Yom Kippur, which will be observed from sundown on Sept. 20 until nightfall on Sept. 21, 2026. Traditionally, fasting on Yom Kippur means forgoing food and water for about 25 hour, which can strongly affect a person’s metabolism.

Other religious traditions include different types of fasts. For example, Muslims who fast during Ramadan neither eat nor drink from dawn to sunset, but they break their fast each night. Christian and Hindu traditions also include dry fasts as well as partial fasts, such as Lent and Navratri, in which people avoid certain foods.

As a specialist in obesity medicine, I work with many patients who take GLP-1 drugs. Most people can safely fast for Yom Kippur or other religious or cultural observances. But depending on their circumstances, they may need to plan ahead, take certain precautions and be clear on when it’s medically advised that they break their fast.

GLP-1 drugs such as Ozempic, Mounjaro, Wegovy and Zepbound mimic a natural gut hormone– glucagon-like peptide 1 – that lowers your blood sugar by triggering insulin release when your blood sugar is high. These drugs also slow how quickly food passes through the stomach, which reduces appetite to aid in weight loss.

GLP-1s have some common side effects that can occur in anyone taking them, including nausea, vomiting, dizziness, constipation and dehydration. Fasting may exacerbate them, especially in people who are prone to an upset stomach. GLP-1 side effects are also more common when a patient is increasing their dose, which usually happens monthly as tolerated until they reach their full dosage.

That’s why, if you’re planning to fast for Yom Kippur, it’s best to avoid starting a new GLP-1 medication or increasing the dose of your medication sooner than two weeks or so before the holiday.

If you take GLP-1 drugs as weekly injections, which is the most common way of dosing these medications, you should stick to your schedule even if you plan to fast. These drugs stay in your system during the entire week, so moving or skipping that weekly dose right before fasting can interfere with their effects on blood sugar or weight loss.

If your weekly injection falls on Yom Kippur, you can generally go ahead and administer it as scheduled. Some patients do report increased nausea on their injection day – if that happens to you, you may wish to delay the injection until after the fast, if you’ve cleared it with your doctor.

Patients who inject GLP-1 drugs daily may need to adjust the timing. For patients taking these medications for diabetes treatment, it may make sense to do the injection right before the meal that breaks the fast or to hold off until after the fast. This helps reduce the risk of stomach side effects, which can occur when these medications are taken on an empty stomach.

More recently, many patients have switched to oral GLP-1 medications. These pills must generally be taken on an empty stomach with a small amount of water, at a set time before taking any other drugs or any food or beverages.

Because fasting for Yom Kippur includes abstaining from drinking, taking the pill as directed may involve breaking the fast. That means you’ll need to decide in advance whether to take the medicine before the fast begins, after the fast ends or on an alternative schedule. It’s best to avoid simply skipping the daily dose scheduled for Yom Kippur and taking a double dose after the fast.

Skipping the dose won’t be harmful if you’re taking the medication for weight loss. But if you have diabetes, skipping it could dangerously elevate your blood sugar. And doubling the dose later could lead to serious side effects, such as pancreatitis, an inflammation of the pancreas that can lead to hospitalization.

If you’re using GLP-1 medications solely for weight management, fasting for 25 hours should not pose serious issues beyond stomach upset. In rare cases, though, combining these drugs with fasting can stress the pancreas enough to cause pancreatitis.

Fasting on GLP-1s carries the biggest risks for diabetic patients. That’s because GLP-1 medications are often used alongside other drugs that work primarily by lowering blood sugar, which raises patients’ overall risk of a blood sugar crash.

The greatest concern is in patients taking a GLP-1 medication together with insulin or drugs that stimulate insulin production, such as sulfonylureas. The GLP-1 medication itself may not cause a drop in blood sugar. But because it suppresses appetite and lowers glucose levels, these other medications can magnify the risk of low blood sugar during a prolonged fast.

If that’s your situation, talk to your doctor about whether you can adjust the timing, dose and type of insulin you take and reduce your dose of sulfonylurea in order to fast safely.

In general, people who have Type 1 diabetes or poorly controlled diabetes are strongly advised not to fast – as are people who:

Regardless of what GLP-1 medication you take and for what indication, if you plan to fast you’ll need to do some pregaming, monitor how your fast is going and be mindful of how you break it.

First, if you are unsure about anything, talk to the clinician who is monitoring your GLP-1 drug treatment so you can decide together whether fasting is safe for you and whether your situation requires any special precautions.

Before fasting, it’s important to eat a normal meal that includes protein and to avoid both overeating and undereating. Overeating is never recommended when on GLP-1 medications, and undereating can set you up for more side effects during the fast. Although GLP-1 therapy can substantially reduce appetite, patients should avoid skipping the prefast meal entirely.

GLP-1 medications don’t just decrease appetite – they can also suppress the body’s natural thirst signals. So before your fast begins, aim to stay hydrated to prevent getting dehydrated while fasting.

If you are diabetic, make sure you fast under close guidance from your doctor – and make sure you are checking your blood sugar during fasting. Using a glucose monitor is a good idea, particularly if you are taking insulin or other glucose-lowering medications. The goal is for your blood sugar to remain within your usual daily nonfasting range.

Be ready to break your fast right away if you start feeling dizzy, confused, faint, nauseated or acutely ill, or if your blood sugar dips too low or spikes too high. These symptoms are dangerous and need to be addressed immediately. Stash a rapid source of carbohydrates – even just some glucose tablets or a few hard candies – within reach so you can access them quickly if needed.

Finally, plan a gentle, balanced meal to break the fast rather than an unusually large meal, which may provoke nausea, abdominal pain, reflux or vomiting. It’s generally best to avoid higher-fat foods, too, since they can slow down digestion.

This article is republished from The Conversation, a nonprofit, independent news organization bringing you facts and trustworthy analysis to help you make sense of our complex world. It was written by: Reed Berger-Fleishman, University of Illinois Chicago

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Reed Berger-Fleishman does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.