GLP-1 side effects range from common digestive symptoms to less frequent problems that need medical attention. Wegovy contains semaglutide; Mounjaro contains the dual GIP/GLP-1 agonist tirzepatide. Their risks overlap, but a finding about one medicine should not automatically be applied to every drug in the group.

Common GLP-1 digestive side effects

Nausea, vomiting, diarrhoea and constipation are frequently reported. Symptoms may ease, but persistent vomiting, dehydration or severe abdominal pain should not simply be dismissed as an expected adjustment. The Mounjaro patient leaflet describes common symptoms and serious reactions requiring prompt assessment.

Semaglutide and the rare eye condition NAION

In February 2026, the MHRA updated its advice on semaglutide and NAION, a rare optic-nerve condition associated with sudden vision loss. That review concerned semaglutide rather than all GLP-1 medicines. Sudden loss of vision or rapidly worsening eyesight during treatment warrants urgent eye-casualty or emergency department assessment.

Mounjaro, contraception and pregnancy

The MHRA’s contraception guidance advises people taking Mounjaro who use an oral contraceptive to add barrier contraception for four weeks after starting and for four weeks after each dose increase, or discuss a non-oral method. GLP-1 medicines should not be used during pregnancy; discuss pregnancy plans and any suspected pregnancy with the prescriber.

Hair loss and changes in facial appearance

“Ozempic face” is an informal description of changes in appearance, not a diagnosis or proof of a unique drug reaction. A change noticed during weight loss needs to be considered in its wider context rather than attributed to a medicine from a social-media label alone.

Hair loss is listed in the Mounjaro leaflet for people treated for weight management. That listing does not identify the cause of every individual episode. Discuss persistent or distressing hair loss with a clinician rather than assuming it is harmless, inevitable or a reason to begin supplements without assessment.

How to assess new side-effect reports

Check whether a report comes from a clinical trial, a regulator’s review or an individual anecdote. A suspected event can be worth reporting without proving causation. Informal labels such as “Ozempic foot” should not replace a clinical assessment of pain or other new symptoms.

Use the current leaflet for the exact medicine, tell the prescriber about troublesome symptoms and report suspected adverse reactions through MHRA Yellow Card.

More science explainers and Svelta Labs information are available at https://sveltalabs.com.

Research-use disclaimer: This article is intended for educational purposes only and does not provide medical advice or treatment recommendations. Always consult healthcare professionals for personal medical concerns.

Sources

MHRA: GLP-1 medicines for weight loss and diabetes: https://www.gov.uk/government/publications/glp-1-medicines-for-weight-loss-and-diabetes-what-you-need-to-know/glp-1-medicines-for-weight-loss-and-diabetes-what-you-need-to-know.

MHRA: Semaglutide and NAION safety update, 5 February 2026: https://www.gov.uk/drug-safety-update/semaglutide-wegovy-ozempic-and-rybelsus-risk-of-non-arteritic-anterior-ischemic-optic-neuropathy-naion.

Eli Lilly: UK Mounjaro KwikPen patient leaflet, revised August 2026: https://www.medicines.org.uk/emc/files/pil.15481.pdf.

MHRA: Yellow Card reporting: https://yellowcard.mhra.gov.uk/.