Tirzepatide (Mounjaro): how it works and what to know

Tirzepatide is a GIP and GLP-1 receptor agonist. The NHS says it can be used to manage type 2 diabetes or obesity, alongside exercise and diet changes. The emc product information explains that it improves insulin release in response to glucose, increases insulin sensitivity, reduces glucagon, delays stomach emptying and, for weight, reduces appetite and increases fullness.

The emc entry sets out the licensed weight-management use for adults with obesity or with overweight and at least one weight-related condition such as hypertension, abnormal lipids, sleep apnoea, cardiovascular disease, prediabetes or type 2 diabetes. Access on the NHS is staged and decided locally, so questions about eligibility belong with a clinician.

It is a pre-filled pen injected under the skin of the abdomen, thigh or upper arm once a week, at any time of day, ideally on the same day each week, with sites rotated. The emc information describes a starting dose that is increased in steps at intervals of at least four weeks. Dose steps are chosen by your prescriber.

The NHS says to inject as soon as possible if the dose is no more than four days late; if later than four days, skip it and return to the normal day. Never double up. Do not stop suddenly without speaking to your doctor; if you have type 2 diabetes stopping could affect your blood sugar, and a gradual reduction may be recommended.

The NHS lists sickness, vomiting, constipation, diarrhoea, stomach pain, tiredness, itchy or red injection sites and hair loss as common. Serious effects include anaphylaxis, low blood sugar when used with other diabetes medicines, pancreatitis with severe stomach or back pain, and gallstones or gallbladder inflammation. The emc information notes that gut effects peak during the dose-increase phase and ease over time, and that vomiting and diarrhoea can cause dehydration that harms the kidneys.

Rapid improvement in blood glucose may temporarily worsen diabetic retinopathy, so eye screening matters. After a dose overdose the NHS says to contact NHS 111 and ask for an ambulance or alternative transport rather than driving yourself.

• Severe persistent abdominal pain: urgent advice.

• Swollen throat or tongue, breathing difficulty: 999.

The emc information reports that a single dose of tirzepatide reduced exposure to the oestrogen in oral contraceptives and delayed its absorption. Because evidence in people with overweight or obesity is limited, it recommends non-oral or barrier methods for four weeks after starting and after each dose increase. The NHS gives the same four-week barrier advice and names condoms, diaphragms or caps.

The NHS says it is not suitable if you have had an allergic reaction to tirzepatide or other GLP-1 medicines, or have had acute pancreatitis. The emc information adds caution in severe stomach emptying problems and says use in people with a history of pancreatitis has not been studied.

The NHS says it is not used in pregnancy because evidence is lacking and advises stopping at least four weeks before trying to conceive. The emc entry says breastfeeding may be considered compatible because levels in milk were very low in a small study, but the NHS page states it is not used while breastfeeding, so ask your clinician. Named interactions on the NHS page include gliclazide, glimepiride, metformin, tolbutamide, warfarin and insulin. Tell anaesthetists you use it because of aspiration risk. Avoid alcohol if it worsens sickness.

It is prescription-only. The NHS page warns that counterfeit products exist, which is a reason for caution about any source outside regulated UK pharmacies. Suspected side effects can be reported through the Yellow Card scheme.