The most common side effects of GLP-1 medicines affect the digestive tract. Nausea, vomiting, diarrhoea and constipation occur in at least one patient in ten, mainly in the first weeks and after every increase in dose, and they usually ease with time. In the STEP 1 trial 4.5 percent of people stopped treatment for this reason. Rarer but more serious are acute pancreatitis and gallstones, reported in 1.6 percent of those treated in the semaglutide trials. With severe abdominal pain radiating to the back the drug has to be stopped and the patient has to see a doctor the same day.
When GLP-1 medicines require caution and adjustments to other medicines
Liraglutide, semaglutide and tirzepatide should not be used during pregnancy. Before a planned pregnancy, semaglutide should be stopped at least 2 months beforehand and tirzepatide at least one month beforehand. Semaglutide and liraglutide should not be used while breastfeeding. A doctor may consider tirzepatide after an individual assessment of benefits and risks; breastfeeding data are limited.
Semaglutide and liraglutide are not recommended in severe kidney or liver impairment or NYHA class IV heart failure. Tirzepatide does not require a dose adjustment for kidney or liver impairment, but caution is needed in severe impairment because experience is limited. A history of pancreatitis requires caution when considering semaglutide or tirzepatide and does not in itself rule out treatment. If a doctor confirms acute pancreatitis during treatment, the medicine being taken when it occurred should not be restarted.
Liraglutide is not recommended for people aged 75 or older. There are limited data on the use of semaglutide and tirzepatide in people aged 85 or older.
In patients with type 2 diabetes who take insulin or a sulfonylurea, the doctor adjusts the doses of those drugs, because together with a GLP-1 medicine the risk of hypoglycaemia rises. People with diabetic retinopathy have to attend follow-up appointments with an ophthalmologist.
Patients taking tirzepatide and the contraceptive pill are advised to add a barrier method as a precaution. It should be used for 4 weeks after starting treatment and after each dose increase. An alternative is to switch to non-oral contraception after consulting a doctor.
Before any planned general anaesthesia the patient has to tell the anaesthetist about the drug, because the slowed emptying of the stomach raises the risk of aspiration. If vision suddenly worsens during treatment, an ophthalmologist has to be seen urgently.
What a patient risks with semaglutide bought outside a pharmacy
GLP-1 medicines are available only on prescription from pharmacies. Poland's Chief Pharmaceutical Inspectorate has already warned about counterfeit semaglutide pens sold outside pharmacies. In 2025, together with Poland's Chief Sanitary Inspectorate, it also warned about semaglutide sold online as a research peptide, without a prescription and with no control over dose or purity. Such a product may contain a different substance, or a different amount of semaglutide, than the seller declares, and the patient cannot check either the contents or the dose at home. We do not treat patients who are taking preparations from sources like these until they switch to a drug from a pharmacy.