Weight loss medicine safety information

Who must not take these medicines #

The UK product documents list one absolute bar for all four medicines: allergy to any ingredient. A prescriber will also refuse treatment or take extra care in the situations below.

• Pregnancy, trying to conceive or breastfeeding.
• Age under 18. Aero Clinic treats adults aged 18 to 75 only.
• Past or current pancreatitis, because none of the four was studied in this group.
• Severe gastroparesis, where food stays in the stomach far longer than normal.
• Severe kidney or liver impairment for the Wegovy pill. Severe liver impairment or the most severe class of heart failure for Foundayo.

Declare every condition and medicine in your health check. See eligibility and the clinical team.

Key warnings compared across the four medicines #

Warnings on low blood sugar, diabetic eye disease and anaesthetics apply equally to all four medicines. The table shows where the documents differ.

Pancreatitis, gallbladder disease and the thyroid #

Pancreatitis

Acute pancreatitis is uncommon, affecting up to 1 in 100 people on tirzepatide or semaglutide. In Wegovy 2.4 mg trials the rate was 0.2%, against under 0.1% on placebo. Treatment does not restart after a confirmed case.

Gallbladder

Gallstones are a known effect of weight loss medicines. In Wegovy 2.4 mg trials, 1.6% of patients developed gallstones and 0.6% developed an inflamed gallbladder. The Mounjaro leaflet lists gallstones as common in weight management use, meaning up to 1 in 10 people. Upper stomach pain, yellowing skin, pale stools or fever are signs to report. See the gallstones guide.

Thyroid

UK product documents list no thyroid condition as a bar to treatment. The Wegovy tablet SmPC records thyroid C-cell tumours in rodent studies as a class effect of GLP-1 medicines and rates the relevance for humans as low, though not completely excluded. Neither the Mounjaro leaflet nor the Foundayo leaflet carries a thyroid warning. Tell your prescriber about any thyroid condition.

Eyes, kidneys and blood sugar #

• Diabetic retinopathy: fast improvement in blood sugar has been linked to a temporary worsening of diabetic eye disease. If you have type 2 diabetes and retinopathy, tell your prescriber and eye clinic before you start.
• Sudden loss of vision: semaglutide documents list a rare eye condition called NAION. Sudden sight loss needs an urgent eye assessment.
• Dehydration and kidneys: nausea, vomiting and diarrhoea drain fluid and can reduce kidney function. The MHRA reports cases severe enough for hospital care. Drink plenty of fluids.
• Low blood sugar: the risk rises when you take one of these medicines with insulin or a sulfonylurea such as gliclazide. Your diabetes team may lower those doses. Never cut insulin sharply without advice, because the MHRA has reports of diabetic ketoacidosis after rapid insulin reduction. Read GLP-1 medicines and diabetes.

Pregnancy, breastfeeding and contraception #

Use effective contraception throughout treatment. If you become pregnant, stop the medicine and speak to a healthcare professional as soon as possible, because safety data in pregnancy is lacking. MHRA guidance says not to take GLP-1 medicines while breastfeeding.

Stop semaglutide (Wegovy injection or pill) at least 2 months before trying to conceive, Mounjaro at least 1 month before and Foundayo at least 3 weeks before.

The contraceptive pill with Mounjaro or Foundayo

Mounjaro can reduce how well the contraceptive pill works in women with overweight or obesity. Add condoms for 4 weeks after your first dose and for 4 weeks after every dose increase, or switch to a non-oral method such as the coil or implant. Foundayo carries the same advice with a 30-day window. Full detail sits in pregnancy and contraception.

Surgery, driving and alcohol #

These medicines slow stomach emptying. Stomach contents may enter your lungs under general anaesthetic or deep sedation, even after normal fasting. MHRA advice from January 2025: tell your healthcare team and anaesthetist before any procedure, and keep taking your medicine unless your doctor says otherwise.

Driving is unlikely to be affected. Do not drive with signs of low blood sugar such as dizziness, sweating or confusion. Semaglutide can cause dizziness during dose increases.

The Mounjaro and Foundayo leaflets give no alcohol rule. The NHS advises avoiding alcohol on tirzepatide because drinking can worsen nausea and vomiting. See GLP-1 medicines and alcohol.

Buying safely and reporting side effects #

The MHRA says the only way to guarantee a genuine product is to use a legitimate pharmacy after a consultation with a healthcare professional. Check the pharmacy on the General Pharmaceutical Council register. Aero Clinic's partner pharmacy: awaiting clinical provider details.

• Do not buy from social media, beauty salons or unregulated websites.
• Genuine products come as pre-filled pens or tablets. Powder in vials for mixing is not authorised.
• Counterfeit pens can closely copy the real product. The MHRA has reports of severe side effects from fakes.
• After the 4 doses in a pen, do not draw out leftover medicine.

Report any suspected side effect, or a suspected fake, to the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk, and tell the clinical team. Common effects are covered in the side effects guide.

Weight loss medicine safety information: questions #

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