GLP-1 Side Effects: A Pharmacist's Guide to Nausea, Constipation and When to Worry

A man in his fifties came to my counter on a Thursday afternoon, holding a weekly pen in one hand and his phone in the other. "Three days after my second injection I felt awful," he said. "Is this normal, or is something wrong?" He was asking about GLP-1 side effects, and I hear that question, in some form, almost every week.

The honest answer is that most people feel some stomach upset on these medicines, most of it eases with time, and a small number of symptoms are warning signs that should never be waited out. This guide sorts one from the other. It covers semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro, Zepbound), the two most widely prescribed medicines in this group.

Why GLP-1 medicines upset the stomach

These medicines copy gut hormones that tell your brain you are full. They also slow the speed at which food leaves your stomach. That is part of how they reduce appetite, and it is also why nausea, bloating and constipation are so common.

The Zepbound prescribing information notes that most nausea, vomiting and diarrhea occurred during dose increases and became less frequent over time. It also notes that the slowing of the stomach is greatest after the first dose and fades with continued use. So the rough pattern is this: symptoms tend to be worst in the days after you start, and again after each step up in dose.

How common are GLP-1 side effects?

The US labels give the numbers from the clinical trials. Here are the figures for the weight-management doses.

| Side effect | Wegovy 2.4 mg | Placebo | |---|---|---| | Nausea | 44% | 16% | | Diarrhea | 30% | 16% | | Vomiting | 24% | 6% | | Constipation | 24% | 11% |

For Zepbound, the label reports nausea in 25% to 29% of people (8% on placebo), diarrhea in 19% to 23% (8%), vomiting in 8% to 13% (2%) and constipation in 11% to 17% (5%), depending on the dose.

Two things stand out. People on placebo also had stomach symptoms, so not every upset comes from the medicine. And only a minority had severe symptoms. In the Wegovy label, severe stomach reactions occurred in 4.1% of people on the injection and 0.9% on placebo, and 4.3% stopped the medicine because of them. These are trial figures, and real life can differ.

What helps with nausea

Nausea is the symptom people ask about most. These steps are widely used, and a joint nutrition advisory for people on GLP-1 medicines supports several of them:

• Eat smaller meals, more slowly. The advisory suggests a small breakfast and then small meals about every three to four hours, with fluids between.
• Go easy on very fatty or greasy food, especially in the first few days after an injection.
• Stop eating when you feel full. The medicine has turned up your fullness signal, and pushing past it is a common cause of vomiting.
• Avoid alcohol. The NHS page for semaglutide notes that alcohol can make nausea and vomiting worse.
• Sip fluids through the day instead of drinking large amounts with meals.

The same advisory mentions ginger or peppermint tea as options some people find soothing. The evidence is limited, but they are low risk for most people.

Please do not stop the medicine on your own, and do not change the dose. The NHS advises keeping on with it unless you are told to stop, and speaking to a pharmacist or doctor if side effects bother you or do not go away. Your prescriber may slow the pace of increases, and the Wegovy label itself says that a dose increase can be delayed if the current dose is not tolerated.

Constipation: the quiet problem

Constipation is easy to dismiss, and it matters. Slower digestion, smaller meals and less water can combine to make stools hard and infrequent.

What tends to help:

• Drink enough fluids through the day.
• Build up fiber gradually from vegetables, fruit, beans and whole grains. Prunes and other dried fruit are a classic choice. Adding a lot of fiber very quickly can cause gas and bloating.
• Move. Even a daily walk keeps the bowel working.
• Ask about a laxative. The nutrition advisory lists fiber supplements, polyethylene glycol (PEG 3350), stool softeners and magnesium as options. Which one is right depends on your other medicines and your kidney function, so check with your pharmacist before buying something off the shelf.

Constipation that becomes severe is a different matter. The Ozempic label lists ileus, intestinal obstruction and severe constipation including fecal impaction among the problems reported after the medicine came into wider use. A bowel that has stopped moving, with a swollen belly and vomiting, needs urgent assessment.

Diarrhea, dehydration and your kidneys

Some people get the opposite problem. Loose stools and vomiting can drain fluid quickly, and this matters more than it sounds. The Wegovy, Ozempic and Zepbound labels all warn about acute kidney injury, and most reports followed fluid loss from vomiting or diarrhea. The labels advise taking care to avoid dehydration, and to report persistent vomiting or diarrhea.

If you cannot keep fluids down, or you are very dizzy, passing very little urine, or feel weak and faint, call your prescriber the same day. If you also take medicines that affect fluid balance or your kidneys, ask your pharmacist whether any need a temporary pause during illness. Do not pause anything without that advice.

Other effects you may notice

• Tiredness. The NHS lists this as a common effect. Mention it if it does not ease.
• Hair shedding. The NHS lists hair loss as a common side effect. It is linked with rapid weight loss, is usually temporary, and is worth mentioning to your prescriber if it worries you.
• Injection site redness or itching. Rotate between your abdomen, thigh and upper arm, as your pen's instructions show.
• A faster resting heart rate. The Wegovy label reports an average rise of about 1 to 4 beats per minute.
• Low blood sugar. On their own these medicines rarely cause it. The risk goes up with insulin or sulfonylurea tablets, and the Zepbound label reports low blood sugar in 10.3% of trial participants with type 2 diabetes who also took a sulfonylurea. If you take these, ask your prescriber whether their doses need adjusting. In the US, the CDC defines low blood sugar as below 70 mg/dL (3.9 mmol/L).

Rarer problems that need attention

The labels list several uncommon but serious risks. You do not need to be frightened of them, but you should know them.

• Pancreatitis. Inflammation of the pancreas causes severe, persistent stomach pain that may spread to the back. The labels say to stop the medicine and get care if it is suspected.
• Gallbladder disease. Gallstones can follow rapid weight loss, and the Wegovy label reports gallstones in 1.6% of people on the injection against 0.7% on placebo. Pain in the upper right belly, fever or yellow skin or eyes should be checked.
• Thyroid warning. These medicines carry a boxed warning because they caused thyroid tumors in rodents. It is not known if this happens in people. They should not be used by anyone with a personal or family history of medullary thyroid cancer or MEN 2. Report a lump in the neck, trouble swallowing, ongoing hoarseness or shortness of breath.
• Allergic reaction. Swelling of the face, lips or tongue, a raised itchy rash or trouble breathing.
• Eye changes. People with diabetic eye disease should be monitored, according to the Wegovy label. The NHS lists sudden vision loss, usually in one eye, as a reason to seek urgent help.
• Anesthesia. The labels note rare reports of food or fluid entering the lungs during sedation. Tell your surgical team you take one of these medicines before any procedure.

A word on compounded and online products

Side effects become harder to judge when the product itself is uncertain. The US Food and Drug Administration (FDA) states that compounded drugs are not FDA approved and should only be used when a patient's medical needs cannot be met by an approved drug. As of May 31, 2026, the FDA reported 990 adverse event reports for compounded semaglutide and more than 730 for compounded tirzepatide. It says these numbers are probably underreported.

The agency has reported dosing errors that led to hospitalization, counterfeit Ozempic in the US supply chain, and products sold as "for research purposes". Its warning signs include deep discounts, no prescription requirement, no licensed doctor to ask questions, and medicine that arrives warm or damaged. The UK regulator, the MHRA, has also warned that powder in vials to be mixed before injection is not authorized, and that legitimate products come as pre-filled pens.

Use a prescription from a licensed clinician, filled at a state-licensed pharmacy (in the UK, one registered with the General Pharmaceutical Council).

Questions to ask your doctor or pharmacist

• Which side effects should I expect, and which should make me call you the same day?
• If nausea or constipation does not settle, can the dose increases be slowed?
• Do I need a laxative or anti-nausea medicine, and is it safe with my other medicines?
• Do any of my current medicines need adjusting, especially insulin, sulfonylureas, blood pressure tablets or diuretics?
• Do I need to take special steps if I get sick, or before a procedure or surgery?
• Does this medicine affect my birth control? (The Zepbound label advises a non-oral method or an added barrier method for four weeks after starting and after each dose increase.)
• How do I tell if my medicine came from a legitimate source?

This article is general information and does not replace advice from your own doctor or pharmacist.

If you'd like the full guide, my book The Pharmacist's GLP-1 Guide goes deeper into starting these medicines safely, managing side effects and protecting your muscle.

Sources

• U.S. Food and Drug Administration. WEGOVY (semaglutide) prescribing information, revised 03/2026
• U.S. Food and Drug Administration. ZEPBOUND (tirzepatide) prescribing information, revised 04/2026
• U.S. Food and Drug Administration. OZEMPIC (semaglutide) prescribing information, revised 05/2026
• U.S. Food and Drug Administration. FDA's concerns with unapproved GLP-1 drugs used for weight loss (updated October 1, 2026)
• NHS. Semaglutide
• NHS. Tirzepatide (Mounjaro)
• Mozaffarian D, et al. Nutritional priorities to support GLP-1 therapy for obesity: a joint advisory. Obesity. 2025;33(8):1475-1503.
• Centers for Disease Control and Prevention. Treatment of low blood sugar (hypoglycemia)
• The Pharmaceutical Journal. MHRA urges women taking weight loss drugs to use effective contraception (June 2025, reporting MHRA advice)