GLP-1 medicines such as Ozempic and Wegovy (semaglutide) and Mounjaro and Zepbound (tirzepatide) are now used by millions of people for diabetes and weight loss. Most people tolerate them, but side effects are common, especially at first. This guide explains which effects are expected, which are rare but serious, how long they tend to last, and what helps.

Why side effects happen

These drugs mimic gut hormones that tell the brain you are full and slow the emptying of the stomach. That is how they reduce appetite — and it is also why most of their side effects involve the digestive system. Food stays in the stomach longer, so eating a normal-sized or fatty meal can lead to nausea, fullness, burping or reflux.

The common side effects

  • Nausea — the most common, affecting up to around four in ten people on the highest weight-loss doses in trials;
  • Diarrhea and constipation, each affecting roughly a quarter to a third;
  • Vomiting, less common than nausea;
  • Heartburn, burping and bloating;
  • Fatigue, headache and dizziness, often linked to eating and drinking less;
  • Injection-site reactions, usually mild redness or itching.

Most of these are mild to moderate. In large trials, a minority of people — generally under one in ten — stopped treatment because of side effects.

How long they last

Digestive side effects are typically worst in the first few weeks and after each dose increase, then ease as the body adjusts, often within days to a few weeks at a stable dose. That is why these drugs are started at a low dose and stepped up gradually over months. If side effects are hard to tolerate, doctors can slow the increases or keep a person on a lower dose. After the last dose, weekly semaglutide and tirzepatide take several weeks to leave the body, so effects fade gradually rather than overnight.

What helps

  • Eat smaller meals and stop when you first feel full.
  • Go easy on fatty, greasy or very sweet foods, which tend to worsen nausea.
  • Drink enough fluids, especially with diarrhea or vomiting.
  • Prioritize protein, fiber and regular movement to help with constipation and preserve muscle.
  • Avoid lying down straight after eating if reflux is a problem.

“Ozempic face,” hair loss and muscle loss

“Ozempic face” describes a hollowed or older-looking face after weight loss. It is not a direct drug effect but a result of losing facial fat, which happens with significant weight loss by any means. Some people notice temporary hair shedding, also typical of rapid weight loss. A meaningful share of the weight lost on these drugs can be muscle, not just fat, which is why eating enough protein and doing strength training are recommended, particularly for older adults.

Rare but serious side effects

  • Pancreatitis: severe, persistent pain in the upper abdomen, often spreading to the back, sometimes with vomiting.
  • Gallbladder problems, including gallstones, which are more likely with rapid weight loss.
  • Dehydration and kidney injury, usually from prolonged vomiting or diarrhea.
  • Low blood sugar, mainly when combined with insulin or sulfonylureas.
  • Severe stomach slowing or bowel obstruction, reported rarely.
  • Allergic reactions, including swelling of the face or throat.
  • Sudden vision loss from ischemic optic neuropathy, which is very rare (see our report on the latest study); people with diabetic eye disease also need monitoring when blood sugar falls quickly.

The thyroid warning

Injectable GLP-1 drugs carry a boxed warning about thyroid C-cell tumors, based on studies in rodents. It has not been shown to occur in humans, but people with a personal or family history of medullary thyroid cancer or the genetic condition MEN2 should not use them. A lump or swelling in the neck, hoarseness or trouble swallowing should be checked.

Surgery, pregnancy and other situations

Because the stomach empties slowly, there is a risk of inhaling stomach contents under anesthesia; always tell the surgical team you take a GLP-1 drug, as they may adjust fasting instructions or timing. These drugs should not be used during pregnancy, and people planning a pregnancy should talk to their doctor about when to stop. Concerns about mood and suicidal thoughts were investigated closely: in January 2026 the FDA reported that a review of 91 trials with more than 100,000 participants found no increased risk, and asked for the warning to be removed from the labels of Wegovy, Zepbound and Saxenda. Any new or worsening low mood should still be raised with a clinician.

When to get help

Contact a doctor promptly, or seek urgent care, for: severe or persistent abdominal pain; vomiting that stops you keeping fluids down; signs of dehydration; yellowing of the skin or eyes; sudden vision changes; a swollen face or difficulty breathing; or a neck lump. Patients have also reported many symptoms online that trials did not systematically record (see our analysis of what patients describe), so mention anything unusual.

A note on look-alikes

Products sold online as “research peptides” or from unlicensed sellers may contain the wrong dose or contaminants, adding risks beyond these known side effects. Use only prescriptions filled by a licensed pharmacy. For most people, side effects are manageable and fade with time; a pharmacist or prescriber can help adjust the plan. This explainer is general information, not medical advice.